I've bought a new motherboard, and will install 8GB of RAM.
I'll install a 64bit linux OS on it, but would like to dual boot with 32 bit WindowsXP for usage of old windows programs.
-Will XP run and just not recognize/ignore the additional ram? Or will I have to limit the system to 4G?
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XP will recognize just 4GB; the 64-bit Linux will see all 8. You should have no problems with this setup. Alternatively, if your Windows programs aren't too demanding, running XP in a VM such as VirtualBox might be a more satisfactory solution.