XP both Home & professional ( 32 or 64bit but 64 is not recommended ) these require sp2 or later
Just a little note on this. XP 64-bit is based on Windows Server 2003 64-bit. Thus, the SP2 for XP 32-bit does not correlate to SP2 for XP 64-bit. So, SP2 for XP 64-bit may not be required (Apple hasn't stated since they don't support it at all). SP1 or no service pack may install fine without having to modify the ISO (slipstream the SP into the iso).
The biggest issue though, is lack of drivers provided by Apple for XP 64-bit. So, some things may or may not work.
With a 64bit OS you can access 4GB of RAM but only 3.3 with 32
It varies from system to system. It's basically 4 gigs minus whatever else is in that 4 gig address space (like graphics card memory, I/O address space, and many other miscellaneous stuff that subtracts from the total amount of RAM that XP may be able to address).
For instance, my Macbook Pro with the 8600M GT 128 MB can detect more memory in 32-bit XP/Vista than a Macbook Pro with 8600M GT 256 MB (theoretically assuming everything else is the same).