Some component is always the limit to framerates. Getting an HD7850 means the video settings will probably not limit framerate for a 1080p monitor for most games. -- that means something other than the video card will be the bottleneck. But thats not a bad thing.
I'd get the card. Game with it. Overclock teh X2 245 if you can and MB/BIOS supports OC'ing. Eventually you'll upgrade the CPU/MB or buy a new computer and move the 7850 into it. HD7850 is a really nice $200 card.