I just picked up a new one myself, a
Litespeed Sabr
Whether you're going to pick up a beater or a bike to do some serious training on it would be a good idea to get yourself into a good bike shop and get an idea for what size bike is right for your body size. Just by shopping and asking them a few questions you will get a good idea of what will work for you, then you can buy there or elsewhere if you want but at least you will know more about what you're doing.
I spent 2 hours getting measured and fit for my new ride when I picked it up. I did a race on it last weekend and it was amazing. Definitely worth the time spent.