I don't know how your area is for used cameras,
But check out the different brands personally. See how they feel in your hands. A camera that doesn't fit your hand, or button positioning that just doesn't make sense to you is a camera you probably aren't going to enjoy shooting with.
Otherwise, it is hard to go wrong with the Canon XTi and 30D or Nikon D70s or D200 for good, solid used cameras.
I do not really know much about the Canon camera line, but I do know that in Nikon, I wouldn't really suggest the D50, D40 or D60. The D50 is a neutered D70 in essence, and doesn't really save up much money now on the used market. The D40 and D60 however, have the decided problem of lacking an in-body AF drive, which kills their usefulness in using Nikon's vast selection of awesome older lenses. One of the lenses that I really want is the 80-200mm F2.8 which requires the AF drive to operate. The benefit of this lens is that it I can usually find it for $800, whereas a 70-200mm 2.8 lens will run me an extra $400.
The Pentax K20D is another solid option, on the plus side, it has great weather sealing for its class. Take it out in any weather, it LIKES it! Pentax also has a great catalog of weather sealed lenses, and classic glass to choose from.
On the minus; It's ISO performance is not great. As well, Pentax glass is just shy of being the same quality as Nikon or Canon. Sometimes having a bit more distortion, or a little more chromatic abberations, etc. But do not take this to mean the glass is bad, on this scale, it's nit picking.
Sony I have no experience in, and cannot comment.
Olympus DSLRs are great for what they are, and what they are is a bit of a niche market. They use a smaller sensor than any other DSLR, with all the tradeoffs that come with it: They are smaller, lighter, more compact bodies. The lenses are smaller as well, and being honest: can be brutally sharp because of this (Smaller glass = less surface area = less impurities/areas for an error). Problems come in ISO performance, depth of field, less wide-angle, etc.
For first glass: I highly suggest a 50mm F1.8 prime lens for any camera you choose. All of them seem to be razor sharp, they are light and fun. Great for portaits and most general photography you will run into. It basically replaced the stock 18-70mm that came with my D70.
After that, try to save up for a 80-200 F2.8 class lens. Available from most manufacturers. I have yet to get one, but my god how I want one. Very useful zoom range, sharp, and useful in most light situations.