I was, until last month, a big proponent of Vista 64. More RAM, supposed better security, and better memory usage.
However, I can't begin to explain the hellacious experience I'm going through right now finding a compatible wireless card. I've tried several D-Links, NetGear, TrendNet to no avail. Have installed dozens of drivers (including several 3rd party)... still no dice. No useful support from any company although all adapters I have used claim "Vista 64-bit compatibility". Considering the dozen forum threads I have seen, apparently, I am not the only one having trouble with wireless networking (and with XP 64).
Long story short, 64-bit is great... unless you want to use wireless. In that case, steer clear.
Edit: Considering my Wii, PS3, Xbox 360, iPhone and girlfriend's MAC and Zune all can use my connection/router perfectly, this is ridiculous.