I'd go with one of the small cards like GT 440 or GT 430. Neither require an auxillary power connector, so fit within the power spec of the PCie bus. Looking at the specs they are both very similar, though the 440 has better clock rates. With only 96 Cuda cores they certainly wouldn't be mega crunchers, but better than the 520, which is lower power & has only 48 cores. Another option is the slightly more powerful GT 545 DDR3, but that may be pushing the power envelope, and perhaps not a big improvement on those two (maybe, maybe not).