Martin Imber:
If you are going dual/quad core, seriously consider going to Vista, and updating your very old apps that may not run (yet to find any myself that are good under xp, but not Vista)
32bit OS can access 4gb RAM. With PC architecture, this is normally capped to 3.5Gb (bios shadowing, option roms, video window etc).
Windows architecture is (as std) 2gb user (per process), 2gb kernel (shared), but a boot.ini switch can change to 3g/1g. Kernel uses little memory, so much beyond 3g in 32bit Windows is pointless (ignoring non consumer, and PAE technologies in servers)
64bit windows overcomes this, each version of (consumer) windows puts artificial limits on. 64bit has trouble with lack of drivers, but Vista is again the best. Older nasty printers are probably biggest issue.
FSB speeds, RAM is slow, not worth spending a premium on faster fsb speeds, ram no longer truely sycronous. DDR2 is quad pumped, but internally cannot deliver.
I know you don't like change and will find any excuse to slate Vista, once you 'understand' it, trust me, it makes sense.
Memory amounts. 1gb min for xp, 2gb min for vista will give speedy system
