I will support 7750 first if you can afford near 100 or a bit less, then 6670, and then all other crap.
Considering you crappy processor, with 7750 you would know that what ever is not performing well, it is not video card to blame. Your maximum processor upgrade is Pentium D 960, which is the thing of long past.
So this is my opinion, now choose model number and I will look for particular one, you need special fit - there are not so many choices outthere, only 2 models of 7750 in USA!