...buddy of mine needs a soundcard.

A couple of ??s

I was considering giving him one of my X-Fi's (Fatal1ty Pro, to be precise) and getting an Auzen Prelude to replace it. The X-Fi works great in Vista64 for me with Creative's latest drivers, so I'll not bash them unnecessarily as is the vogue nowadays.

Is the Auzentech Prelude a good upgrade for a Vista64 platform from an X-Fi? ...or should I stick with the X-Fi...or would I be better off with a Asus Xonar? TBH, I enjoy Creative's software console but I think the Prelude uses the same thing. Doe the Xonar...

1) Sound as good, work as well in Vista64?
2) Have an equally usable software interface in Vista64?

They are all pretty much equal. X-Fi's perform better usually. Why not get your buddy a X-Fi XtremeGamer, last time I checked they were $60.99 after $30 rebate. I believe free shipping too, from Newegg. That way you can keep your X-Fi.

The Prelude from reviews it supposedly a little nicer when repoducing music through quality speaker system. This is due to the higher quality OPAMPS and other components used in the Prelude. However, it supposedly falls behind a little in EAX duties. Prelude will have DDL, where current creative product i believe you need to pay for that priveldge. The Xonar is not up to either the Prelude of X-fi in music or EAX situations (only emulates EAX 4 and 5).

Personally id take on pcgamer12's suggestion and just get a X-fiGamer for you mate and stick with what you got

Okeydoke, I'll stick with what I've got and perhaps get him an XtremeGamer.