Check first if your motherboard supports 8GB of ram. Some motherboards physically limit the memory to 4GB or less. If you install 8GB and your motherboard is hardware limited to 4GB, when you get to Win7, the other 4GB would appear as "Hardware Reserved" and be unusable.
There would be no problems with WinXP with massive amounts of ram, it would just happen that it can't "see" the ram past 3.3~4GB.