a.) like what baller said, buy your own parts at www.newegg.com (best choice)
b.) if you don't to (it's really not as hard as you think it is) build your own computer, try:
www.monarchcomputer.com <--great site, i've bought great stuff from them before www.ibuypower.com <--you can configure your own comp there including the case and psu and they build it for you
c.) visit a local store, they usually have good prices (although they charge a lot for building)
i've heard of it and seen it in pc gamer mags but i wouldn't recommend them since i've heard that they have bad customer service/have poor cable management
but then again, that's just me
there's their ratings right here, you decide for yourself if you want to buy from them - i wouldn't:
