Go for it. Games like BF2142 fully support it and it sounds awesome. Having a card like an X-Fi Xtrememusic takes strain off the cpu and can give you a slight fps advantage. I see no reason not to get an X-Fi because you can just carry that over to your new system when you start over again.
Stacker 830 - e6600 @ 3.6ghz - Eva 9800GTX+
X-Fi Xtrememusic - 4GB OCZ SLI 1066
Sceptre 20.1" - Logitech Z5500 - Gigabyte DS3 965p