Building/Buying a gaming PC

For the HDD: Go with the the Caviar Black 640gb http://www.newegg.ca/Product/Product.aspx?Item=N82E16822136319&Tpk=caviar%20black%20640gb
The reason is it has 2 platters vs 3 when you compare it to a 740gb. Less platters= more speed
For GPU: I say switch for an Ati because you won't be able to put 2 Nvidia cards in that board for future use. Ati Future use= get another card when you need more performance Nvidia Future use= buy a brand new card and let the old one go to waste
Just remember to look at a PSU review

Edit: Whats with the Retail OS?

Why do people keep recommending the WD blacks when they're slower and louder than the samsung F3s?

The HD103SJ uses 2x500 GB platters while the WD blacks you recommended use 2x320GB.

And not to turn this into a long debate, but he can handle 2x GTX460's on a 650W PSU. The GF104 core found in the GTX460 runs cooler and more efficient than the GF100 core found in the 465/470/480.

If you're worried about SLI, there's a hack for AMD boards.