I would not upgrade, just get an entire new system based around an i5 2500k.
Toms conveniently put up an article answering that very question:
http://www.tomshardware.com/reviews/...mark,3120.html
They used a 7970 to test a bunch of AMD/Intel chips to see if there is bottlenecking and yup the AMD chips bottlenecked it, pretty badly in some cases. The games used to test aren't particularly well optimized games and BF3 is done in single player but they are popular games for sure even on the PC.