I run 8gb with XP 64 at home. At work 11gb on XP 64. We'll be upgrading to 8-core systems with 16gb of RAM soon here at work too.
Dude, 64 bit driver issues? Hardly anymore. If you would have asked me 3 years ago what I thought of XP 64 I would have said DONT DO IT.
Driver support is excellent anymore.
And yes, I believe 4gb is better than 2gb for 32 bit windows. It's not really wasted per say. It gets kind of complicated addressing that remaining memory. You can flip the /4gb switch - google it.
IT's a waste not having 4gb of memory. It's dirt cheap anymore! You can pick it up some places for like 20 bucks after rebate, or 35 with free shipping no rebate.