I believe you need a low profile card which will usually only take up one slot and it seems the best card you can get is likely an AMD HD 7750 low profile card, but such a card will not play Starcraft 2 at max. But that is the fastest card that fits that size that I know of. This is assuming you are using a Small Form Factor PC case.
There are low profile cards of the 750 and 750ti which outperform a 7750 but I wouldn't recommend any of these on that psu. A 730 is all I would put on that and you're looking at high at best.