if you have dual channel or ganged memory (quad core) stick 4 gig in, i use 2 x 2 gb sticks on my 32 bit vista set up it shows 3356 ram in task manager and seems the happiest set up i have tried on that pc. as ventanas says with sp1 it shows all 4 gig in control panel, system, it still can't use anymore, because it's 32 bit.
2x 2Gb is ideal, as adman says. RAM is inexpensive these days - the difference in price between 3Gb & 4Gb is probably ~£10. Plus, this gives you easier room for expansion greater than 4Gb in the future should you ever need it (mobo needs 4x RAM slots). G