If you're thinking about piecing together a PC.. the three sites I would reccomend (in this order) are:
NewEgg
TigerDirect
PriceWatch
Just before Christmas Tiger Direct had a deal on a complete system, only thing missing was a monitor I believe.. anyway, it was like a 2.4GHz or something with a good size hard drive, they were selling it for $450, with $200 or $250 in rebates.. best deal I'd seen in a long while. But, between the three it's pretty easy to piece together a top of the line, or damn close to it, PC for about $600.