Well if you're trying to future proof this computer , I'd go with a quad core, such as Q6700, if you can manage to fit that in your budget. Otherwise you can get the Q6600 and overclock it
.
Again, if you want to future proof, you'd want to go with a P35 motherboard, otherwise go with something like Asus P5B-Plus or Asus P5B Deluxe.
For the graphics card, I would go with the 8800gtx. Don't go with two lower cards in SLI, because that doesn't get much performance increase over a single card...plus the power requirements are nasty for SLI.
I would also suggest 2Gb of ram..such as
these.