to answer the original question.
first, it does not matter to the OS what the memory type is: DDR, DDR2, DDR3, etc... it is transparent to the OS. there is no such OS that requires a specific type of memory
Windows XP SP3 32-bit can run just fine on 4gig memory, only that some of the memory are wasted. the the amount of the "unseen" memory may vary especially if you have a dedicated video card. if your video card has 1GB of dedicated memory, the total RAM that can be utilized by windows will go down to less than 3GB for a 4Gig-main memory system... this is the limitation of any 32-bit operating systems.
32-bit version of Windows 7 and Vista is affected with the same memory limit just as windows XP.
there is workaround for this(32-bit), google for PAE, but it is not very helpful either.
a single 32-bit windows Application can only utilize up to 2gig of memory regardless if windows sees 3.5Gb or more
to fully utilize memory larger than 4gb, use a 64-bit OS. ex. Windows XP x64, Windows Vista 64-bit, Windows 7 64-bit, 64-bit Linuxes, MacOS X Snow Leopard i think.
32-bit windows 7 or Vista can run with as low as 512mb of main memory. don't start arguing.
it is not that you need at least 4gb of memory for a 64-bit OS like Windows 7 64-bit or Windows XP x64, it is, however, capable of utilizing 4gb or more.. 64-bit applications are also required.
hope this make things a little clearer. don't be compelled by the maketing hype.
you don't need a 4gb for just browsing the web reading this. maybe in the future... but i still doubt since a it is very hard to make a 4gb webpage
