Stick with either Nikon or Canon, there's a reason all the pros use either - they're very modular, reliable systems. For example, you can mount any lens from 1959 until 2009 on the Nikon D40, which is probably the cheapest Nikon DSLR. I happen to like Nikon better, but canon is fine too. Try both companies' cameras out and see which brand feels better in your hand. For Nikon, if you're interesting in buying used, I recommend (in this order): D80, D50, D200, D70/s, D40, D60, D40X. If you're buying new, you should get the D90 (or the D3000 if you can't afford it, MAYBE the D5000). I have the D300 and it's a lovely cam, the D90 is similar in a lot of ways. The best Nikon kit lens is the 18-70, WAY better than any 18-55 or 18-135, and probably on par with the 18-105. You can find it used for around $150-200, last I checked.
For Canon, the 30D is a great camera. The 40D was pretty much a flop because of the screen so it was replaced with the 50D, basically the same thing with a better screen and more megapixels for more money. The 30D is older now but still a good camera for under $500 used. I'd also recommend the XTi, or if you're buying new, the XSi. The XS is just too small and the T1i is a bit too expensive. The Canon 18-55 is a bit better than the Nikon IMO but it's not great and I don't know of a nice Canon normal zoom, other than the 17-55 ($700ish?). The 17-85 is okay and cheaper, but also a lot slower.
Anyway hope that helps. You really need to go to best buy or target or wherever and try out some cameras though.