The i7 920 is the smart choice. It has plenty of power for any task you can throw at it. And later when you do need more power you can always "dabble" with some overclocking
i7 Overclocking Guide
Asus, EVGA and Gigabyte are the most popular mobo choices.
Look for a 6GB (3x2GB) kit of DDR3 1600
At this very moment in time the ATI 5870 is a much better card than the GTX 285. Expect Nvidia to release a few new cards in the very near future.
Graphics Performance List
Your probably not going to need a 1000watt power supply anytime soon. And a smaller wattage unit like the Corsair 850TX will perform as good or better than a "so-called" 1000w unit from a company like Ultra or RaidMax.
*READ FIRST* Updated Power supply guide
Cooler Master, Antec, Lian-Li all make excellent, well ventilated cases.
You should get yourself a nice LCD Monitor. I will also say a 24" minimun preferably HD with a 1900x1200 resolution. Samsung is an excellent choice.
Unless you specifically need XP mode for a specific application (not for gaming) then you should probably look at Windows 7 Home Premium.
The best Heatsinks for Air Cooling are, in my opinion, the:
Thermalright True-120 Extreme, this heatsink needs lapping to bring out all of it's glory. See the i7 Overclocking Guide link above for more details on lapping. Push-Pull is an option.
Prolimatech Megahalem is also a very good heatsink that can be setup in a push-pull configuration.
Noctua NH-U12P SE2 is another good heatsink and it ships as a push-pull heatsink straight from the box.