I haven't read all the other posts, so sorry if a repeat.
Go to a bike shop, and get fitted. Yes it's considdered rude to get fit, then buy somewhere else, but... I have a deal where all my maint. and most major parts purch, come from that shop that hooked me up. You can also ask them to hook you up with something used, as when your ready to go big, you'll come to them. They may hook you up expecting your addiction to increase...
Go to bikes direct .com and you can find some pretty good deals. Try e-bay first as they will blow out last years models for cheap. I bought an entry level carbon and 7005 alm. bike with 105 components for $750 shipped to my door with the pedals (read ok, but not the best pedals). The frame says Windsor on it, but it's made by TREK.
Good wheel-sets, good tires (kevlar), and a good fit are important. But pretty much anything over your '70's bike will feel like a rocket.
Wheel and deal, get the word out at the good bike shops, alot of people are trading up right now. If you talk to the mechanics, and they are asses, say adios. There are many bike shops that won't,and don't like to work withnovices looking for deals. A good bike shop, with nice people make you want to go back..Good luck, biking is absolutely delicious. Just finished a quick 20 miles (of hills) this morning with a friend... you just can't beat the work out, and it's fun too.