Yeah the GPU is a bigger deciding factor. What is it?
A better CPU would also help too, isn't the 9500 one of the ones with the L3 cache bug? They had to release a hotfix that would hurt performance by about 10%(?) in some parts. And I think games are starting to use L3 more too. I'd try and upgrade to another quad core, the Phenom ii quads are literally light years ahead of the Phenom i. You can get the 955 on ebay for the same price as the 455 in the UK
You'd need to check board compatability, if your board won't support the newer Phenoms you'd probably be stuck with the 9950. Its stock speed is what your CPU is overclocked to, it doesn't have the L3 bug and It will go to 3ghz, so It'll still be an upgrade, but I dunno whether it'd be noticeable. It would definitely be noticeable in BF3 64 player multiplayer, that game is very CPU intensive. But you'd have to be savvy on ebay, they can go for stupid prices. The 9850 is only 100mhz slower and much cheaper most of the time.
If you get a Pentium you would have to buy a new board and new ram. Intel CPUs use an entirely different socket, LGA1155 and use DDR3 ram instead of DDR2.