The answer is: in most games, no FPS difference.
About 10 years ago, when 3D accelerating cards were far from ubiquitous, many games offered software rendering. That is, if you lacked a card that supported OpenGL, Glide or Direct3D, you could still play a game at very low quality settings using CPU alone. That optiion is now gone - game will do what it can with your video hardware, but no more.
It is kinda similar with sound these days. Games for the most part will make use of a n EAX card if they find one, but if not, you simply get basic sound. Not that many games DO bother with advanced sound that could use an EAX card, truth be told. And only a handful of games (Crysis is one, I believe) offer advanced sound via software - that is using the CPU. So, if you are playing one of those games, and chose the 'advanced sound via software', and have a monster video system that is constrained (or close to being constrained) by a sub-optimal CPU, then a dedicated sound card will increase performance. Lots of ifs here.
Real reason for an X-Fi card is quality of sound. Not the 'audiophile' type quality, but rather the sound environment. I.E. Fear, Thief 3, Bioshock all sound incredibly more detailed with an X-Fi card. The card can resolve a lot more sources at once, with proper application of distance and occlusion.