- I'm not sure about the graphics card and mainboard as there are a lot of different suppliers : Asus, Sapphire, MSI, XFX, ... There's also a TOP model from Asus and a 2GB version, ... difference with a GTX275 with less memory?
- Is there additional cooling necessary?
If you're planning on crossfiring in the future, get a higher rated power supply, most likely a 750w Corsair/PC Power and Cooling/Antec. The VaporX 4890 by sapphire is rated very high for performance and quiet.
I agree with the PSU recommendations, and will also add OCZ to the list. Large amounts of GPU memory is needed for driving multiple, or large displays at high resolutions. If you don't plan on hooking it up to a beast of a monitor, extra GPU memory won't give you more performance.
I was actually referring to dirtmountain's comment on PSU suggestions, OCZ makes good quality power supplies at a fair price. In their memory department, they tend to have better timings, though it's not a huge difference, and only becomes important if you plan on overclocking everything. Here's the OCZ RAM similar to the kingston you selected: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Timings are a tiny bit better, purchase price is lower. Though with the $50 rebate, the Kingston is cheaper. I'd say it's a toss up, pick which ever you like more.
There isn't a whole lot out there on how DX 11 will perform, but I can say for sure that games that will fully support DX 11 will lag behind the card releases by 6 months to a year. That's how it went from DX 9 to DX 10. Any new games you will want to play in the next 6 months will look and play just fine on DX 10, so I wouldn't make direct X support a major factor in deciding between high-end mainstream cards.
Core I7s run hot to begin with, the stock heatsink/fan cooler that comes with the I7 is adequate if you stay at stock speeds. If you want to overclock at all, get an aftermarket cooler and good thermal paste. The i7 cooler from Xigmatek seems to be the best value and easiest to install.
The HD you have listed is the slower green edition. The green drives are nice for HTPC's and as storage only drives, but not fast enough to be the main system drive in an i7 build IMO. I would swap it out for the faster black edition drive for an extra $10.
do all the kits have the same voltage ratings? if not, get the one with the lowest voltage. It will run a bit cooler, and marginally help keep your system from needing higher fan speeds. There is a performance difference between the CL7 and CL9, but it won't be very pronounced unless you overclock your system.