erm...he never asked about the barrier, nor is he even using a 32bit OS.
he's using vista premium 64bit, which can take upto 16gb of ram.
anyway, in reply to the origional post,
vista works beutifully with 4+ gb of ram.
while true there wasen't as much of a performance gain when going from 2 to 4gb as there was when going from 1 to 2gb, the gain is still very noticable. especially when multitasking. games run great, the pc is faster due to less hard drive access (less need for a pageing file, which really speeds up games)
with vista, its definatly worth having 4gb of ram.
and, i'm saying that from experience.