Just upgraded from onboard to an x-Fi, and CMSS (most notably in BF2) is surprising, almost frightening. If you can find the x-Fi for around $70-$80 (which it routinely is after rebates or during sales), it's definitely worth the money (from onboard). For retail price of $130, however, I wouldn't think so.
I have a pair of Grado SR-60's I use for gaming, and after reading the sparse, yet mostly negative reviews of 5.1 headphones, opted to just add an external sound card instead.
PM