I believe that an aero helmet give you more of an advantage than wheels. Its also cheaper.

Helmet, then wheels if $ allows.

While doing some research on aero helmets, I ran across this info on helmets.org. Updated 1/07:
Chrono Helmet Models

# Bell Meteor II: designed by adding foam under the Bell Meteor to meet the CEN standard, so it is limited in size to 59 cm heads and does not meet the US CPSC standard and will not be available in the US market. Weighs 315 grams.

# Briko Chrono: has two large rear vents and a modest tail that is more rounded than sharply tapered. It has a plastic face shield. It is certified only to the European EN 1078 bike helmet standard, so you can't buy it in the US but it would be accepted by the USA Cycling for racing here. Weighs 409 grams/13.1 oz. We don't see the Chrono in Briko's 2007 catalog, but assume it is still available.

# Carrera Intruder: classic teardrop shape but with two panels of stippled material glued into indentations on the surface. Two top vents and a tiny vent on each side.

# Casco Warp II This 2006 German helmet is an almost perfectly round and smooth track sprinter's helmet with an above-the-nose shield completing the rounding. The shape is flawless for crashing. Casco claims it is equally flawless for aerodynamics "according to the latest findings of the automobile industry." This seems like a reaction to the aero tails that have set the fashion in chrono and other high end bicycle helmets for the last decade. It has no vents, costs 250 Euros and only meets the CEN standard.

# Catlike Chrono: Catlike has a new chrono model for 2007. It is molded in the shell and designed to meet the European EN 1078 bike helmet standard.

# Cratoni Chrono: Short shell does not fully meet the shoulder or back. No vents. Certified only to the European EN 1078 bike helmet standard. Weight is cited as 270 grams, if accurate very light for a chrono. Fits heads 54 to 60cm/21.3 to 23.6 inches.

# Giro: Giro has a time trial/pursuit helmet that according to Bicycle Retailer and Industry News will not be sold to consumers. Lance Armstrong and the Posties used the Giro Advantage and Rev VI time trial helmets in Tour de France time trials. The Advantage is now available to consumers in Europe, but the Rev VI was designed only for Lance. Press reports indicate that the Advantage was designed for the European CEN standard. It is a long tail design open underneath the tail that meets the rider's back if the rider is in correct position. The Rev VI has a lump in the back. Both helmets have small slit vents and the underside of the tail is open.

In 2007 Giro launched the Advantage 2 in the US market. It meets the CPSC impact standad, and is molded in the shell with five small vents. It retails for $250.

# KED Zeitfahren: KED's chrono model comes in long and short versions. The short version is called the Track and looks like a regular bike helmet but is smooth-skinned with no vents except in the rear. The long version is the Time Trial and has a long tail that will extend to the rider's back. It has no vents. Both have CEN certification only.

# LAS Chrono: Long polycarbonate shell, no front vents and an integrated clear partial front face shield. Very long tail to reach the rider's back with a slight shoulder hump. Certified to both the CPSC and CEN standards. Ring fit for 54 to 61cm heads. Retail is $240. Trialtir's catalog photo shows the rider with the tail sticking up in the air.

# Lazer Chrono 3: Lazer's version of the chrono shape has a short tail style. It is molded in the shell with unfortunate external strap anchors, three small vents and a ring fit system. It has both CEN and CPSC certification. Weight is a light 300g. Retails for $190.

# Limar Chrono 05: Limar's 2005 aerodynamic pursuit and time trial helmet with CPSC certification is molded in the shell with small vents in the rear and three unfortunate lumpy external strap anchors. The shell is long to reach the shoulders. Face shield optional. Limar says the short shape permits more efficient bike position and works better when the rider is out of aero position or standing. Weighs 330g. Ring fit. Retails for $200.

# Limar Superchrono: Limar's CEN-only chrono with ring fit system and a taped on shell has two large front vents. Not for the US market. Pricing is described as "affordable."

# Louis Garneau Rocket: A 2005 chrono design from LG, the maker of the first chrono meeting the CPSC standard in 2002. This is Garneau's second generation, and there are improvements in the form of a longer, slicker shape with a full shell covering all the impact foam, a curved buckle that fits facial contours and can be manipulated with one hand, and four small vents in the front. Retail price is $180. The windscreen is available separately for $30.

# Louis Garneau Chrono: Very similar to the original LG chrono model, in a classic teardrop shape. Has three small vents. Retail is $90.

# MET Pac III: This Italian company has a long-tailed design with two vents. It weighs 290 grams and comes in two sizes to fit 53 to 59 cm heads. MET's helmets are not available in the US or Canada.

# Rudy Project Syton Supercomp: Another Italian company, with a chrono model with more vents than most, a face shield and a split tail. Molded in the shell. There are unique pieces on the sides that extend down to about the cheekbones, called "bionic wings." Medium length tail does not get all the way down to the shoulder. Comes with plugs for the front vents. Ring fit with two shell sizes to fit 54 to 61 cm heads. Comes in red, white and blue as well as white. CPSC certification. $200.

# Rudy Project Syton Open: the Supercomp without side pieces or face shield, retailing for $160.

# Selev Chrono: a unique design with an all-carbon fiber shell that follows the external line of the Selev Alien road model, but has a full lower cover that comes all the way down to the neck. Meets the EN1078 standard.

# Shain BK500: actually a road helmet with an additional fairing added on the exterior and a clear face shield. There are three small front vents, five total. Meets the CPSC standard. Normal EPS foam with Shain's inner shell, weighing 350 gr. It retails for $197. Certified to the CPSC standard.

# Specialized: The Specialized chrono model does not appear anywhere we can find in retail or catalog litarature, except for a catalog photo of the Cofidis team wearing them. Press reports indicate that it has larger vents than other TT helmets. The tail is open underneath and the fit system is based on the Specialized Decibel road model. Rumor has it that it is available from some dealers in Europe, and that they are saying it meets the CEN standard.

# Spiuk Kronos: Spiuk's chrono model comes in the classic teardrop shape with two small front vents and a rear vent through the tail. Ring fit. Meets the US CPSC bicycle helmet standard. Appears to be a fairing that was put over a normal EPS foam liner. Retail is $230.

# Uvex Factory Pilot 2: A classic chrono design with a long tail that lies flat on the riders back and is covered on the bottom. Has low sides, and a nicely integrated face shield. It has an Acoustic Warning System that hums when it is off-center to alert the rider to less-than-ideal aero performance. Weighs 280 g. Meets the European EN 1078 standard but not CPSC. Retails for $500, the most expensive chrono we have seen, but Uvex says it is not available through retail channels. Look for it on the T-Mobile TT team.

LG Helmets can also be customized here: http://www.louisgarneau.com/usa/ctm_catalog.asp?catalogue=C7&section=HE