Purchased/built a homebuilt right when the dual core came out, and now with college behind me and money to spend, as well as Diablo III coming out soon enough, going at it again.
Approximate Purchase Date : 1 Week
Budget Range : ~ 1500 $
System Usage : Hardcore Gaming
Country : USA
Parts Preferences : Nvidia / Intel / Gigabyte
Overclocking : Not off the bat
SLI : Not off the bat
Gaming builds don't even use up 4GB of RAM so 16GB is overkill. I'd recommend saving the money by getting 8GB and putting that saved money into getting a GTX 680 (or 7950 or 7970) - all of which outperform the GTX 580.
A 750W PSU will be borderline enough to run a system with two GTX 580s and I would advise against it, however it is plenty for a system with two GTX 680s, so that is another reason not to use a GTX 580.
The WD 1TB FALS isn't worth any amount more than this, which is both faster and quiet/cooler:
Seagate Barracuda 1TB ST1000DM003 $110 http://www.newegg.com/Product/Product.aspx?Item=N82E168...
(Even the 2TB version is cheaper than the 1TB FALS)
You could even use a 5400RPM drive if you are not going to be putting any programs on it.