What camera should I buy?

I'm 16, & will be starting photography cles at Durham Tech. I have recently been taking some portrait's of friends & family, for a little publicity. Right now I just have a point & shoot, but would like something nicer preferably a Nikon or Cannon. Cheaper is better, but I want something with good quality. 8 mega pixels or more & I want to be able to use a external flash unit & ball head tripod. Something in the range of $400-$800. Any photography tips are appreciated also!