Well I am not a software developer but I can tell you from my professional experience in IT, that 64bit is not anywhere near being a standard platform for windows. Developers have not released support for it yet. Vista has been in production for over 6 years and released for over 9 months and driver support still sucks for it. Having over 3.25gigs of RAM does not even improve system performance unless you are actually using all of it. Also, since Vista is also the biggest flop of upgrades and major companies like HP and Dell and Lenevo have decided to let their customer's get XP on their systems still and people are still gettting it, there is no pressure on the developers to even start making 64bit applications.
If you can find drivers for the sound it should work, it may work without them. I think you get better reception with a PCI card with the large antenna and isn't pci faster than what a usb could allow? But i think the usb one will be fine, you probably won't be able to tell the difference. If it don't work then you could consider a new card.
best of luck
http://forums.linksys.com/linksys/board/message?board.id=Wireless_Adapters&message.id=2699
Apparently, that has 64bit drivers for it?