Building a computer is fun and all, but for the price, U cant really go wrong with this setup:
http://www.newegg.com/Product/Produc...82E16883229013
In fact, if you price this system out seperatley with the same specs, down to the case, it will cost you more.
With this type of setup, you'll be set for almost anything, plus it has plenty of room for expansion.
Add a nice 22" monitor for about 250-300$ and a nice laser mouse and you're set.