I dunno to be honest, i'm on 32 bit and will probably stay till I need to get a new PC. I can see the advantage of 64 bit but I dont think that I would really use it to its full potential.
I think if you have 4 gig it does see it all, it just keeps a portion to run windows on and diaplays whats available for programms to use, so that would be an advantage but I've only ever had a problem with memeory when I was being stupid like trying to render a head of hair/model with SSS, FG HDR etc etc all in one shot, once I borke it into layers there wasnt a problem.
If youve got the hardware and you dont mind shelling out the cash for the OS and the software, then why not go for it!!
"No pressure, no diamonds" Thomas Carlyle