You can find G.Skill DDR3 1600 MHz with lower CAS latency for about $25 more than the RAM batuchka has listed, though. Very slight difference in performance, but these will run on lower voltage with better latencies at higher speeds.
But yeah, since you still have $50 left in the budget to get up to $850, I would suggest getting these:
Since his resolution can get up to 1920x1200, the benefit from a more powerful CPU may be somewhat diminished, but I'd still be more comfortable recommending the Phenom II x4 build. It comes in under budget, includes a 5850 and will be useful in games that benefit from more powerful cpu cores.
Definitely agree with the G.Skill RAM swap, though.