CPU or GPU

[P373]

I'm looking for an upgrade as i have a bit of money coming in for birthday in about 2 weeks and I was thinking of buying either a q6600 or a HD4850 (similar price) My current rig is in sig and i was wondering what you think would be a better upgrade? I mainly use my rig for cod4 and supcom. The hd2900 can pretty much handle anything I can throw at it without AA (bar crysis) and I fancy a new cpu to mess around with BUT the hd4850 looks like an amazing card for the money.
Thoughts?

 

[P373]

cheers for the replies, now its just deciding the brand. The ASUS, powercolour and HIS are the cheapest but I'm unsure which one to go for? I know the asus one with the custom cooler is on offer but i won't be purchasing it in time. Anyone got any recommendations on brand/reasons?