When I say "problems" I mean as int he card don't work no more...I've had two X-Fi's (gamer and platinum) and both took a dump on me after a month or two.
As for sounding better, the difference is negligible IMO. Better? Sure...but not worth the $100+ you're paying, plus the crappy service from creative. You won't notice much (if any) noise from current onboard sound. It's not like the old mobo's where the onboard sound was hideous, at best. My mobo has Realtek ALC883.
Of course getting a good sound card will yield VERY noticeable results, but I can't say the same for X-Fi's. I will say that earlier soundblaster cards are much better then the X-Fi, as they don't have the popping issue, and just all around sound better. Even with all the Crystallizer crap and such turned off, the sound I got was...lame. I use XP Pro, as well, so it wasn't a driver issue.
I have SR125's, BTW. And I also use an X-Meridian sound card. It certainly yields better sound then onboard.