I'm unclear exactly how far you've got to.

 

The Linksys website might suggest that drivers are not available for Vista x64, but I myself am running Vista x64 on a laptop with a Linksys WPC54GS NIC with excellent results. (I appreciate that you're not using the same h/w, I do have a WMP54G PCI WNIC somewhere but have yet to test it on a Vista x64 machine).

 

So if you are thinking that this is a showstopper to block adoption of Vista x64, I would say go for it. Just ignore what it says on the website, and just give it a go, and just let Windows Update install the drivers. I's certainly not a reason not to go for Vista x64, in any event, as if you should get really stuck, you could always use a wireless-to-ethernet bridge product, such as the WET54G say, instead of a PCI WNIC.