For the most case even 3gb is fairly meaningless for 32bit windows. Each 32bit application can only use 2GB of ram, its a windows limitation. But if you have two applications that both want 1.5GB of ram then all will be used. So it really depends multitasking yes get ram. Single application.. minimal gain.
Of course windows itself uses some ram, but my old 32bit XP used around 256meg of ram including virus scanner, and firewall, so going from 2gb to 3gb, would only increase the ram for a single application by 256meg.
64bit Windows has far less restrictions, and depending on how the applications are compiled there are far more options. 32bit applications on 64bit windows can be compiled to request 4gb memory space each, 64bit applications have virtually no limits.
Vista64 does still have various limits, depending on which version you have installed. With Vista64 having the highest limit at 128gig.
Vista Home premium allows 16gig, and that should be easily achieveable on a Nehalem system with 6 slots
