<*SIGH*>
Not this hot potato again.
OKAY, I'm only going to say this once.
32bit XP DOES support 4GB of physical RAM - whether it's PC3200, PC6400 or whatever.
However, in all systems, some memory addresses are 'mapped' to IO (meaning they're used to communicate with any peripherals, video card, sound card, and do on).
So, because the 32-bit address space is exactly 4GB, if you install 4GB of physical ram, the IO mapped addresses will hide some of the physical addresses.
How many addresses are hidden in this way varies from system to system, so there is no single maximum amount of ram that will be available to windows.
In fact, if one were to build a system that didn't use memory-mapped IO, windows would see all 4GB - but that's only in theory - all modern PCs use memory-mapped IO.
Now to answer the OP, yes you'll see some improvement going from 2GB to 4GB physical RAM, but not as much as you get from going from 1GB to 2GB, unless you do a lot of graphics/video/audio editing, or other tasks that require large amounts of RAM.