The ram you picked is for a 17-1366 build, you need a dual channel kit, in other words 4gb.
The 275 isn't a great choice right now, its high in price/preformance. Look at the 5770/4890/5850.
The sound card is not really needed, the onboard sound is great these days.
For the PSU, if you go with one of the cards I recommended above and will not crossfire, you can get a 550w PSU.
With the introduction of the ATI HD 5000 series GPUs, ATI has effective control of the entire market from the $150-$600 price range.
Doesn't mean you still can't find decent nvidia cards for a cost-effective price; however, the HD 5000 series are great buys right now. With your budget you should be able to afford an HD 5850, which has plenty of juice and supports Direct X11.
i5-750 with the 1156 architecture is the better performance/price match for gaming. The 1366/i7 combination is still slightly better, but in my opinion is not worth the cost (you'd do better do invest that money in the GPU). Hyperthreading doesn't do a whole lot for games, so there's no point in spending the extra money on the i7 (both the 800 and 900 series). The i5 still overclocks like a champ.
Your mobo is good, but check out the GA-P55A-??? series. These have Lotes sockets rather than the Foxconn sockets (Good for heavy overclocking, otherwise not really a big deal) and support USB 3.0 and SATA 6 (however, due to the P55 architecture, the board cannot support SLI/Crossfire and USB 3.0/SATA 6 at the same time).
In any case, the Gigabyte P55A series are about the same price, so as long as your friend has just one GPU, he can also support USB 3.0 and SATA 6 when he needs to. Not a bad buy at all, just not worth it if you see him using dual-GPUs in the future.
For just one GPU, 550W should be enough. The OCZ Fatal1ty 550W has a great mail-in rebate which puts it around $40.
I find Corsair Dominator to be overpriced for what you're getting in terms of performance. It is (no doubt) great RAM, but it's so expensive. Check out this nice G.Skill kit: (http://www.newegg.com/Product/Product.aspx?Item=N82E16820231279). Good speed, tight timings, low voltage requirements, reasonable price. Doesn't get much better...
The 8GB limit in 64bit Windows is only the Home Basic edition.
W7 Pro has a RAM limit of 192GB.
I'm also in agreement with nofun. ATI has nVidia beat in almost every price point across the board at this moment in time, and the i7 isn't much better than the i5 for gaming, but is quite a bit more expensive.
Yeah they got those questions covered, about the sound card...I think you read about nvidia's CUDA technology, in which some of the load is taken off the CPU by the VIDEO card. As they said, ATI is dominating right now so buying an nvidia isn't worth it.
I would stay with the i7-860 specifically to keep hyperthreading which does increase performance on other applications and it is a natural opportunity for games to use it to allow increased performance.
And I still favor nVidia cards for much the same reason - the CUDA feature mentioned above which ATI can't come close to matching. Some games take advantage of it now and more applications are being written for it. That being said - it is true as the others have said that ATI has the lead in most ways. But if you can afford an nVidia card that outperforms the likely ATI pick, why not go for it and have the advantage of CUDA for whatever its worth?
Also you might consider this better case which - because of a pre-Black Friday sale costs about the same as the one you selected:
This is true. nVidia CUDA and PhysX are two technologies that ATI just doesn't have an answer to; however, at present, only a few games actually take advantage of these benefits. The last I heard, the GTX 300 series was scheduled to be released in Q1 2010. These GPUs will support DirectX 11 along with the rest of nVidia's great technology, and will directly compete with the HD 5000 series.
At present, ATI has the better price/performance matchup for gaming.