menetlaus :
Hard to say, I don't really see any specs there on the PSU.
If the PSU has little to no room for upgrades - you might be limited to a 750TI.
With a little more headroom a 265 or such is possible.
With an upgrade to the PSU (my guess) you could go up to a 980, though that might and up bottlenecked by your CPU depending on what you've got.
+1 to that. Looks like the Nvidia 750 Ti would be the most I'd put in that system without upgrading the PSU. The PSU specs are listed below, depending on which form factor the OP has:
Mini-Tower:
305W Standard PSU or optional 255W 88% Efficient PSU; ENERGY STAR® compliant, Active PFC
Desktop:
255W Standard PSU or optional 255W 88% Efficient PSU; ENERGY STAR® compliant, Active PFC
Small Form Factor:
235W Standard PSU or optional 235W 88% Efficient PSU; ENERGY STAR® compliant, Active PFC
Ultra Small Form Factor:
180W up to 90% Efficient Internal PSU, ENERGY STAR® 5.0 compliant, Active PFC