I had the same problem.... I am 5 11, and have long legs and a short torso... some shops can measure you and have software that they can exactly compute what you should get. I suggest calling around to see where you can go for that. From what I'm told, the most important thing is FIT. I was going to buy a bike from ebay, but considering that some people still have old bikes, but insist on keeping them just because they fit so well.... I think you should definately get fitted first, then bid.
Also, just because they say it's a 56 cm or 58 cm... all brands run their sizes differently, the cm is just ONE of a few measurements that are vital. The 56 cm in a Trek may be different in the 56 cm of a Felt. The Felt is priced alright, though. You could also go to shops that have that brand and find out what size you are in that brand. Some shops let you test ride them, but there's no telling how you will feel 50 miles out when you tested the bike down the street! :) Good luck with your choice!

Take a look at Slowtwitch's bike picker: http://www.slowtwitch.com/mainheadings/prodreview/tribikes/bikepicker.html
While there are certainly more choices out there than it suggests, it give you a good idea of what to look for in a bike.

need some major help here i`m very short 4`10 or so 650cc wheels, 48c bikei need a cheap very aero frame that prefferably carbon or aluminum any suggestions, thanks