1. The big question is was the difference in sound quality from the change in headphones, or the change in sound card? I know going from my Corsair HS-1 to my Audio-Technica ATH-AD700's (plus Zalman clip-on mic to make it a "headset"
) on onboard sound (well, the HS1 was USB, so it had its own "sound card", but close enough) was a MUCH larger difference than going from the audio jacks of my P8P67 Pro to my Xonar DX (with the A-T headphones). That said, the DX was definitely an improvement, particularly for listening to music (sounds slightly clearer/cleaner) but not as much as changing headphones was. Point is... getting a sound card probably won't do much on a cheap/crappy set of headphones/speakers.
2. I'm pretty sure to gain FPS from using a soundcard, the game/software has to support hardware acceleration. Even then, the FPS difference would be fairly small.
That said, a sound card can be a nice addition if you have the extra money for it. It should improve overall sound quality, and will have some gimmicky effects to play with (simulated 7.1 surround headphones, etc), which may or may not be good (I know BFBC2 sounds pretty cool with "7.1" enabled, wouldn't say it sounds exactly like sitting in a room with a real 7.1 surround setup though)