Mountain Bikes huh?

Ive gone out a few times now with my friend and ridden his (way too small for me) Gary Fisher mountain bike, and I have decided I like it enough to buy my own.  Started looking around $300 range for a hardtail, now ive upped it to about $450 and Im considering a full suspension.  Checking one out tomorrow on craigslist, hopefully I can bring it home and start riding it.  Only problem is i only like going downhill fast,  going up sucks!  having a proper sized bike might help if when i pedal my knees dont hit my chin.

Here is a picture i found of the bike im checking out tomorrow....its beautiful!