Your processor is fine for COD games..and i would upgrade to 4gbs of ram as well since that will make sure you have enough and release any bottleneck you might have been having by overloading the ram....
The 4770 will give you a HUGE boost in performance IF your resolution is 1440x900 or 1280x1024
If you have a higher resolution you might want a bit more than the 4770
Ok....with this new card in my post and the on you linked to you can play in alot higher resolutions than 800x600 so you are going to want to put your monitor to its native resolution...tell me what that is...and based on that we can recommend you a card...
Dunno what to recommend. The GTS250 is a great single GPU since it is nothing more than a 9800GTX+ that is a tad shorter.
You didn't mention Power Supply type. But your max resolution is pretty low and ANY of the above cards will give you playable frame rates as a single card, you don't need Crossfire for that resolution. Your best bet (if your power supply can handle it) is to buy the fastest Single card, the GTS250.
Ok my psu is unknown standard 700watts power supply came with my prebuilt system tested with a radeon 4870 X2 and performed without issues also i may also go to hi res and preffer ati cuz AMD fan so whats better Xfired 4670 vs 4830 (i ALSO USE MY PC FOR ANTI APPLE USING neighbor bragging to stop his mouth