In short, NO operating system running 32-bit can use more than 3Gb, and using memory extensions doesnt allow the OS to actually use it, but only to detect it or the extension only allows certain applications to use it and performance can even get worse with these softmods.
So if you use Win XP home, pro, Vista home, premium etc doesn't matter, the only thing that matters is if you are using 32/64bit.
And if you have vista 32bit, then you can use the 32bit key to activate a vista 64 bit of the same version, premium/ultimate.
Which you can either order, or just download a disc image from wherever you prefer.