I'm new to these forums and biking in general. I'm going off to college next year (Cal Poly SLO) and would like a bike for getting from class to class as well as riding some light dirt paths. My budget is around $200 - $300 so I suppose buying new is out of the question. I've been searching CL and found a quite few bikes I'm interested in but want your help to narrow it down a bit.

Trek 7200 - $175
Trek 7000 - $180
Trek ZX 6500 - $225
Trek 6000 - $250
Trek 4900 - $275
Trek 8000 - $290
Gary Fisher Mamba - $200
Gary Fisher Marlin - $200

I know that's a lot but hopefully you all can help me out.