My advice: if you're going to do a mid-range performance/value build, spend most of your budget on parts that will carry over into your next build, and that will also benefit your system the most:
All those parts are critical to the performance and reliability of your system, and they won't go obsolete any time soon.
and maybe the videocard too. Paired up with a nice CPU like the i5 2400, you shouldn't get any bottlenecking for games. I'd look at the 6950 1 GB, it can be had right now for around $230 after rebate. It's a huge part of your budget, but spending a $30-50 for a better GPU can make your system carry well into several generations of games.