"Also do you believe that running my firewall, norton are having a detrimental effect on my games?" No, though Norton is renowed as a resource hog.
A new cpu will not make enough of a diff. to justify the cost.
You have plenty of ram, though BF2 does like loads.
I think your best bet is either get another 7800gt and run sli, or get a new card, one of the 7950GX2 cards (effectively sli in one card) that's around £350-400 click here
Don't bother with the physx cards yet, wait till the dust settles.
Hope this helps.