m going to now shoot a LOT of holes in this build.
Firstly DO NOT GET A QUAD (KENTSFIELD)
1. For a start; currently you will not really notice a large difference
2. They produce horrible amounts of heat; and to overclock you wont get anywhere more than 300Mhz out of it with safe temps on air; even with a top 5 heatsink
3. Their Bang4Buck Is absolutely terrible for most things (other than videoencoding)
Therefore go with a dual core like the e6420/6600/6700, you can overclock it past 3Ghz and get a new one in a year or so when the quads are less dodgy (believe me I have a bunch).
Then Go with a decent cooler; if you want to go for air cooling setup then go with a Sunbeam Tuniq Tower 120 (I have a couple), or an Thermalright Ultra 120 (or extreme edition 120).
If you get a half alrite chip you should be able to do 3.4Ghz easily on that cooling and it will work out cheaper, quieter and better for your pocket.
For the Mobo Id go with a P5K (Deluxe Version if you want), which I have and it overclocks like a monster - its the only board ive seen do 8x525fsb on a quad (extreme cooling) - which will be useful for when you do decide to go quad...)
For the Ram, Get 2x 2gb Sticks - Id recommend ballistix or dominator and get a ram fan (if you arent planning on OCing much then get some 2gb OCZ 800 Sticks). It leaves ya more room to upgrade in the future.
Dont Get an 8800Ultra, I have tested some and they are a waste of money for the persformance gain over a GTX, you can pick up a GTX for £300 now, but Id go with a good brand such as EVGA (avaible from scan), as you can always stepup to the ultra at a little later date if you have the inclination.
Also what resultion will you be playing at?
If its 1680x1050 or under you can get away with a GTS 640 with ease, but if you want go for the GTX
HDD Wise Id go for the new 750GB caviar as you have saved a bundle by chaging to these components, its a blisterinly fast drive. Or if not a 1TB Hitachi (32MB cache) is just as good.
PSU wise Id got with a Corsair HX620W, it seems to power dual 8800GTX's and a loada other stuff no problem, and is quiet to.