I currently run vista Ultimate 32bit with 4gb (2 x 2gb) installed. Of course, like everyone else, my system only sees just over 3gb, a fact I was well aware of before buying.
I've had Vista since day 1 and was nervous about previous flakey 64 bit support, but it really does seem to have improved enormously now. I use Lightroom (2.0 now) and Photoshop quite often with heavy, intensive files, and can quite often pretty much eat up 3gb.
I would like to move to Vista 64 bit, but to be honest, as I'm getting by, I've decided to wait until Windows 7 is out, and then make the move to DDR3 on a new platform as well. If I had my decision again though, I'd have started with Vista 64 bit, and put up with a few early niggles on driver support.