The OS is the limit.
32bit Windows does not see all the 4gb of RAM.
Although 64bit Windows technically can see all that RAM it has problems with
stability, program and driver compatibility. Win64 will run Photoshop but
many if not most users cannot use that much RAM effectively.
Vista64 will not be any different, at least at outset, than Win64 and
probably have even less driver support for things like printers, scanners
and calibrators. In fact there will be inadequate driver support when Vista
32 is released in a few weeks.
The truth is that users may think they need that much RAM but the reality is
somewhat different despite what some computer enthusiast magazines/web sites
tout.