ok do not get the x-fi audio!! it's just a software card, ignore it.
personally i have an x-fi music (gamer is just a low profile version)
and if it works with your board it's great, beware problems with nforce4? motherboards. google around for compatibility problems.
if you have good speakers the x-fi gamer is a good card. also for 5.1/7.1 (7.1 you'll need their cables probably, it uses 2x4-pole and 1x3 pole jacks to provide the 8 outputs for 7.1
another advantage is it supports eax 5 which can add so much more to the sound experience of games that are developed with it in mind.
if you couldn't care less about audio then onboard is probably fine, or an old audigy may suit your needs if you want that bit better.
as for vista i think x-fi has reasonable support, and with an x-fi you get alchemy software free which lets you have the eax effects in vista through openal conversion. (the vista driver model pretty much kills normal eax)
AMD athlon 64 3000+, 1GB ddr400, 720GB total hdd, ati radeon X1900gt 256mb (pci-e) 17" tft(@1280x1024).