As said,u will need a 64bit OS to read more than 4gb of system memory.
Remember, system memory means Ram, GPU, Sound. Lan etc. They all count.
Id choose W7 as a Windoze OS. XP64 just never took off. You will find drivers impossible to find.
64bit h/ware is not an issue. If you have a Mobo that can support 8Gb, you will have 64bit compatible h.ware.
Remember, most windows software is only 32bit. So games for example wont be able to make use of any more than ~4gb anyway.