Best graphics card for E2140 (Dual Core @ 1.60Ghz)

I have a really old system with an E2140 (Intel Dual Core @ 1.60Ghz). 1Gig of ram.. using integrated graphics (mirage 3)

I want to get the best graphics card my system can handle without the CPU creating a bottleneck and restricting the graphics card..

i would like to get a HD4850 but suspect my system will hinder its performance..

I'd mainly like to play online shooters like Battlefield Heroes, Combat Arms, Left4Dead, etc. I'm pretty much on the lowest detail settings they will go at the moment.

anyone have any idea how i can calculate this... or what graphics card i should go with?