With a budget of only $300 you are going to have a tough time even in the used market finding a DSLR. It can probably be done, but more realistic for a low end setup capable of all around shooting as well as concert stuff is probably more like 400-500. That said, I'd suggest if nothing else save up a few more $$ and go with a DSLR. You will be much happier in the long run with the control and quality and lens options you get from DSLRs.
Beyond that, the advice above is great. Nikon or Canon are the biggies, Brian just started a great thread where his ultimate decision was Pentax. I'd suggest a quick read through that thread, keeping in mind his budget was slightly higher. Either way, the Nikon D50/70 or the Canon XT/XTi are very nice cameras and readily available in the used market and prices are approaching your stated budget for a body only. Watch out for the Nikon models such as the D40 and possibly D60 (I'm not sure on it) that do not have an on board focus motor, they are good cameras, but limit the lenses you can use with auto focus.