My current system is a 22"(1680x1050), 4850 512mb, pentium dual core E5200 2.5ghz @ 3.25ghz oc, 500w psu with 2x 12v rails(17a and 18a, don't know the brand) but im not satisfied with the performance(im an enthusiast) because i can't max out the graphics (and AA) on any of the following games without lag : Fallout 3, Far Cry 2, Crysis(obviously), Gta IV, Assasins Creed.
from what ive heard, all of them support quad core(correct me if im wrong but i think fallout 3 only support dual and not quad) so i should gain a huge advantage in the games i play if i upgrade to a Q8300, but i should also gain a big improvement jump with a 4890.
Im confused with what to buy(i've also checked the Q9300 but the 2mb extra cache doesn't give enough improvement over the price difference, correct me if im wrong)
I'd try (temporarily) overclocking your CPU more. I've had a E5200 up to 4GHz without much effort.
This might give you an idea if you're GPU or CPU bound... if your games speed up, maybe more CPU horsepower is needed.
It's worth remembering that the Quads are generally clocked lower and overclock less that the dual cores, as getting rid of the heat can be problematic. This probably wouldn't be a problem for games that can utilize the extra cores, but games that are limited to 2 cores might run slower when you move to a lower clocked quad.
A single 4850 is a good performer, but it sounds like you are looking for something mind blowing. A 4850 won't get you there, even with a quad core. To be honest a 4890 will give you better performance, but honestly I'd go for a direct upgrade to your current card, which would be a 5850. This will probably give you the best performance increase. Granted a CPU upgrade will allow your current video card to run better, but it won't be drastic. Likewise if you DID get a 5850 and later upgraded you CPU, you would also see an increase.
GPU upgrade = best gaming bang for buck
CPU upgrade = increased performance, and more so with higher end GPU's