pointless. the 520 will cause your 6770 to bottleneck you need a coda core thats as fast in both gpu and gddr5 memory as your 6770 the amount of cuda cores isnt that important as your only calculating physics with it. but its better to have a card with at least 128.
for my 5870 i would need a minimum of a gtx 260 to run physx on my machine without it bottlenecking. your likley to need a minimum of a 98gt or a gts250
i got only three option with cuda core on newegg
so what about 430 it have 96 cores but it is for pci which is far more slower port
also the thing is i am not sure if this setup can do a post or not, maybe some of you can help me to do that
another question is ringing bells in my head, that is
will it help my cpu to share phyx computing load after selecting gtx as a phyx engine, and thus cutting down the use of my old cpu