980 = 6 cores 3.33GHz/3.60GHz max turbo (+266)
970 = 6 cores 3.20GHz/3.46GHz max turbo (+266)
960 = 4 cores 3.20GHz/3.46GHz max turbo (+266)
950 = 4 cores 3.06GHz/3.33GHz max turbo (+266) (notice a trend?)
Specifically for gaming? Get the 960. All of these CPUs turbo up depending on heat load, and the 4-core chips generate less heat, keeping them at the higher multipliers for longer. Especially since he's getting the 10% factory overclock. He's not going to notice the -4% (133MHz) difference in speed, but he
will probably notice the -$320 difference.
The liquid cooling system will fail eventually. Make sure he is prepared to deal with that inevitable fact. Air coolers are noisier and not as efficient, but they are
much easier to maintain.
I'd upgrade to one of the >1000W Gold-rated PSUs, like the TPG-1050M. Gold-rated PSUs have high efficiency and will actually use
less power than the store-brand 950 when not at max load. Also, the GTX590 is an absolute
monster when it comes to power draw.
A single GTX580 should make him just as happy but with significantly less cost and power draw (meaning less heat and lower electric bill). And yes, dual 580's in SLI are noticeably faster (and hotter) than a single 590.
I'd change the Mobo to either the ASUS Rampage III Formula or the Gigabyte X58A-UD5, just because I know those two brands are the two that everyone gets compared to, and those specific boards have lots of slots/connectivity/overclockability/etc.
All of that said, I still feel like I just helped someone pick out the best DVD player while surrounded by Blu-Ray. If he is
really serious about looking for a gaming rig, he'll
abandon the whole idea of an X58 build and go instead with something based on Z68.
--Patrick