XCOM: Enemy Unknown and Enemy Within are fantastic games. However, even with their mediocre graphics, the game just isn't cut out for the PS3's hardware. Every little motion, ranging from changing mission control screens to shots fired, are very laggy, and load times are incredibly long compared to that of PC play. These things just make the game not nearly as enjoyable on PS3 as it is on PC--even if your PC isn't that great.
Maybe I'm just being biased since I have already played it on the PC, but my recommendation is to skip getting this game for the PS3, and have fun with it on a PC.