The thread has been answered, but I'll just addon... for fun..
I run a GT 240 for PhysX, which I only really bought because I got it for $65 after rebate and I figure, at this point, why not. I already spent a couple grand on my PC, what's $65 to get that last bell? Hah. Anyway, I like it. Played the Mafia 2 demo last night, it looks pretty cool aside from some annoying glitches/crashing, and not the best PC options (although it has great multithreading).
So... yeah. If you want PhysX just for shits and giggles because you play (or will play) Batman AA, Cryostasis, Mirror's Edge, Dark Void, Mafia 2 (and a few otheres) then get a cheap card. A GT240 is great, or go older like 9600GT, 9800GT w/e. Just don't spend a lot you don't need a powerful GPU for it.