@OP: Upgrade your CPU to a Core 2, or a cheaper variant of it (i.e. Pentium Dual-Core), and overclock the snot out of it. This way it would get rid of most CPU bottlenecking. Be sure to get a new PSU as well, a 400W is definitely NOT enough for the 4850. @JN821: As for the person going from 8600 GT to HD 4850, and getting lower FPS, it's usually by PSU, because it can no longer cope with the extra load (600W should be enough, but it doesn't hurt to try a higher quality one to make sure). Someone else on this forum had the same problem, and after he upgraded to a higher quality, higher wattage and amperage PSU, his frame-rates sky-rocketed.