You'll never use 4gb even when running a 64bit edition. For the average desktop 2gb is generally adequate. There's a lot of farse about needing 4gb, 6gb, 8gb installed unless you are planning to run some intense programming like CAD development.
When going from 2gb to 4gb of memory on the build virtually no difference was seen performance wise in anything! When having first gone from 1gb to 2gb on an older build you could readily see XP at the time smooth itself out. Vista later ran quite well thank you.
With Vista seeing SP1 for the 32bit editions some 3.58gb is seen available for Windows and programs over XP's 3-3.12gb limts. The other 500mb is simply mapped out to hardwares as far as memory addresses.
The caution about 64bit editions while some rave they have their favorite games running is that you still can too easily run into problems with other programs and drivers for addon devices at times. If you have an older dsl setup the 64bit drivers may not be available as another thought there.