The best value for the money, shown to provide most frames per second per dollar spent videocard is the ATI 3850. Judging by what you are playing and your computer specs, I'm sure this is all you need.
You can get it for $179 at Memory Express, $10 cheaper than NCIX.
http://www.memoryexpress.com/Product...19168(ME).aspx
If you truely want to future-proof yourself though, you could go all out and get an 8800GT but they range from $259-$330 when not on sale.