I recommend against Mt. Gox simply because they're the biggest exchange, and thus, a single point of failure.
BTC-e can be a lot of fun, and has a good deal of variety with alt-coins, however getting USD into BTC-e is rather difficult, unless you're in Russia. The exchange fees are low over there, but their withdrawal fees also kind of ridiculous - .01 BTC, .5 LTC. I don't have much experience with other exchanges.
It wouldn't hurt to look into #bitcoin-otc on Freenode, and localbitcoins.

If I were you, I would also consider selling goods for BTC on Bitmit - commerce makes for a much more sustainable growth model than everyone trying to buy coins to resell them for more money down the road.