all good advice above, however:
I don't know of bugs on xfi (others may have them, I dont) as my system has been stable and solid since the first day. Sound quality is great, I use good headphones and mediocre speakers.
Yes, you might notice a slight performance improvement in games but not enough to justify the cost... plus as stated before w/ your vid-card you are not gaming so even less justification.
Sound quality is always better on better speakers, (here comes the justification) but even on low-end logitecs I notice background system static w/ onboard sound. (have built many systems w/ it, and have used the soundstorm [great chip] on my asus nforce2 board and tested the sound on my current system) W/ all the extra stuff going on in the mobo there is too much static that crosses over to the sound. On some boards (including the great soundstorm) I can even hear the mouse movements... and that is on a cheap-a$$ pair of $10 logitec headphones!
Not sure if my ears are just hyper-sensitive, but that alone is reason enough to get a sound card. And if you are in the market for one, you might as well make it an x-fi. The quality is there, and it will last you for years to come through many new systems. (heck, I still have an audigy in an old system and it is better [interference wise] than any onboard sound)
Yes, the audigy2 is also a good card, but for just a few bucks more you get the higher end that will last longer. JMO