if you take a look at the cpu-bench at anandtech, and compare athlon2 x4 645 to a 3570k, there are few game benches at the bottom of the list and the athlon gets like 50% of the fps the 3570k gets. I dont know the gpu used in those test but likely it has been quite powerfull one at the time they did the testshttp://www.anandtech.com/bench/Product/188?vs=701
there are other games in the database as well but not every cpu has been tested with every game and certainly not with the latests gpus. for example Crysis Warhead has been tested using a gtx280http://www.anandtech.com/bench/CPU/50
so you might be able get something like 40-66% of the cards performance depending on the games and settings used. However if you plan to upgrade the rest of your system pretty 'soon' to match the cards potential, it might still be a good choice.
(the gtx280 is listed in the same tier as the current gen gtx650 in the toms hierarchy chart http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
7950 offers like 2.5 times the performance of the gtx650 http://www.techpowerup.com/reviews/XFX/HD_7790_Black_Edition_OC/26.html
so there most definitely is the potential for a massive bottleneck with your current cpu.)