HD 5850 is the best price/performance card that you can buy today.
Like the others, we need more info about your current specs...
ATI 5850 and GTX470 are good options for up to $350.
heres my specs: i7 920] rosewill 750w p6t deluxe v2 western digital 15000 velociraptor current VGA is his radeon 4870 ice-q turbo but it freezes some times and makes combat arms' charcter/people/zombie lag like hell. and it was not cheap...180$
Get the HD5850.
+1 jyjjy: Just installed one in my new (now complete) i5 build and it really flies!
BUT... You might want to do more troubleshooting on the HD4870 before splashing out on a new card. What have you tried to stop the freezing so far?
tuning up the memory clock to 1015mhz in catalyst. i got the latest version btw do u play combat arms? it works fine in all my other games exept combat arms.