For Canon equipment, I like KEH or fredmiranda.com's for sale forum. You can score some pretty great deals there. I know booty's all about the pentax so I'll let him cover that. Speaking from experience, moving to a prosumer body was very rewarding. I wholeheartedly support the decision to get a 30D or XSi and a nice med. telephoto.

I think 20D's can be had for like 250 and a good sigma/tamron 17-50 2.8 should be like 350. That would be a great all purpose setup.

And yes you can make the argument that some bodies can use old MF lenses. You can also argue that you can buy 25$ adapter plates to get those to work on all other bodies as well. Not endorsing or supporting one or the other. Just my 2 cents. The equipment is only one component of great photography.