Not all of a given card's RAM is mapped to memory for I/O purposes - up to 256MB would be a more typical number, regardless of the size of the card. For a single GPU install, I wouldn't worry about it either way. Though if Crossfire/SLI is in the build plan, I recommend going 64 bit.
Regarding 64 bit OS's and driver problems: It was somewhat true in the XP64 days, though absolutely do~able with just a little bit of planning. Less True in the early Vista days (though cried about to a *hugely* exaggerated level), and now that we've has several years of Microsoft enforcing their rule: "No Windows Logo For Anyone Without Both x86 and x64 Drivers", it's become pretty much a non-issue for new builds. The exception being if a given user/build has old parts/devices they can't bear to live without.