I also hope the OP understands that its only a Creative-based soundcard that's going to offload the CPU and get him/her that coveted extra 3-4fps. TBH with today's CPUs and GPUs I'm not sure how important that extra 2fps really is, like Stranger said, that's personal preference. I'd say the latest renditions of EAX would really be the gaming reason to go with a Creative card. If music and movies are your thing there are A LOT of good choices out there. Many believe the X-Fi to be the best card out there. However, when you read the reviews often-times you'll see that they're reviewing the X-Fi Elite Pro version which is considerably better than the Extreme Music, Platinum, and Fata1ity versions in terms of DAC, THD+N, and S/N ratio...but at $400, it damn well should be, eh?
Unless you really want the latest EAX, and the other toys the lower-end X-Fi's bring to the table you can get a wonderful soundcard for a lot less $$ than you'll pay for an X-Fi. However, if you go for an X-Fi, you'll have a nice Jack-of-all-trades-Master-of-One.
If you've got onboard sound with a digital output, its a different story. In my personal experience I've found my onboard sounds fine when connected to my Z-5500's via Optical. ...but it should. If I were a gamer though, I'd still just use a Creative card, why take any performance hit at all? Why lose out on EAX 3-5? Why put so much into my rig just to neglect audio? I've yet to hear analog output from onboard sound that competes well with even an old SoundBlaster Live! and I'm using a fairly contemporary mobo with contemporary onboard audio chipset myself.
...but I imagine if you have poor speakers analog onboard sound is just fine.