Radeon HD 5850 bottlneck

Hi,

I recently purchased a radeon 5850 and I think it may be bottlenecking as it has a tendancy to stutter and lag when playing some graphicly intensive games (GTA IV, MW2, starcraft 2 etc) on high at 1440x900. It seems to be especially prevalent when the game first loads. From what I read about the 5850 this should not be the case? So I'm thinking it is bottlenecking somewhere. I may have read that the BIOS of the P5Q could be bottlenecking (again I'm no expert), but am a little apprehensive to update them as I've been told to only update them if I'm having problems (which I'm not sure is the case). From what I could gather my PSU is fine, however I guess I could get another couple of gig of RAM. So I don't know if something is wrong of if I'm just expecting too much. I

Here's my system specs.

Windows Vista Home premium 64 bit
Asus P5Q motherboard
Intel Core 2 Quad LGA775 Q6600 2.40GHz
Radeon HD 5850
2GB Corsair PC2-6400C4 DDR2 Dual Channel Kit
Corsair HX 520W ATX Modular SLI Compliant PSU
Seagate Barracuda 7200.10 320GB SATA-II 16MB Cache

Any help is greatly appreciated.

Thanks