CORE 2 QUAD Q9550 2.83G 12M is good choice but its price is pretty high (around 600$) .
i suggest to go with Core 2 Quad Q6600 (2.4 Ghz 266*9) this processor has almost the half price (u will save over 250$).
from the performance side : The Q9550 is good for 3800 Mhz ++ , The Q6600 can do 3.4-3.6 with some good cooling .
-1) wait until Q9550 get back to normal price (around 300$)
-2) buy now the Q6600 (with option to update later) .
No one will pay additional 300$ for 400mhz or lower :-) .
-ASUS P5K Premium is also good choice , Asus do always well with intel processor , such board is good for 450 ++ (FSB) .
-about the memory , there is a huge of price drop for DDR3 memory . but DDR3 will require expansive chipset such as i790 ,x38 or x48 .The performance between DDR3 and DDR2 is very tiny .
-ABout 8 GB memory : only few motherboard are capable to run 8 Gb without long story , Get some decent DDR2 800 (4-4-4-12 1.8 V) 4 gb witch more than enough for today game .In any case you may update later and add 2 x 2 Gb .
-The card is also good but :
1) wait until the 9800Gtx release (1 april) then Nvidia should drop the price of old generation card .
Here u can add the 200-300 $ saved on processor to get a nice 9800 GX2 .
-Psu : i will go with 700-800W true power PSU (maybe later sli update more hdd etc)
- HDD get the best Gb/$ hard disk . most should perform about the same . if you have heavy disk task u can go with raid 0 using 2x750 GB disk .
U are always welcome to ask more