You can get wheels a lot cheaper than Zipps, but you'll still need to spend some fairly serious money. First off, I'd make sure that you really need them. Aero wheels won't benefit you significantly unless you're averaging over 20 mph during a race.

If you're already averaging that high, or think that you will be soon enough to make spending the money worthwhile, check out used wheels on ebay. You can get some pretty good deals, and the kind of people who have aero wheels generally take very good care of them, so they're in great shape. If you want to buy new, Flashpoint is made by Zipp, but a lot cheaper. I've never ridden them, but I've never heard anything bad about them.

If you're not quite up to 20 mph yet, the biggest difference you can make is to get some stiffer wheels. Something like a Ksyrium SL or ES (anything less, and you're getting pretty much that same as your Bontrager Race) will transfer power better and will be noticeably faster. You can get a good deal on those guys used also, since retail is $850 and $1150 for those two I think.

Consider an aero helmet. You'll get more aerodynamics at a fraction of the cost of race wheels.

Here's teh Bicycling mag article wiht the interbiew of the MIT cycling team and some of their findings.

http://www.bicycling.com/article/0,6610,s1-3-12-14995-1,00.html

i agree with vanjames and triguy. aero is aero. and an aero helmet will help more than wheels if aero is truly your goal. or an aero seat post. i saw a wind tunnel study somewhere (sorry don't have the link handy) that indicates the benefits of a aero seat post and helmet -- they outway that of wheels. so, if were to look at the best return on investment i would buy a seat post (@$200) and helmet ($150) before wheels ($500 ++). but, again, aero is aero so any improvement you can get, no matter your ability, go for it!

Aero wheels are faster regardless of average speed. Aero helmets do provide a less expensive first option for getting more aero, but aero wheels, especially discs, provide a significant improvement.

You can save about a minute with a disc and aero front over 40k. That's usually more than the difference between winning and not winning in an Olympic race. That doesn't mean that aero wheels will automatically make you drastically better, but when you do a bunch of things that all save about a minute (better wheels, better transitions, etc), they add up quickly, so why not take advantage of them all....wheels included?

you best bet is most likely to be some deep dish carbon wheels. I have the Bontrager Aeolus wheel set and they fly. They are cheap too. I would steer away from the disk at this stage as you need to be quite strong to stay up on top of the gear required to keep the "fly wheel" effect happening. As a newb they are hard to drag up hills and can ruin your run legs.