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On a side note...is your BIOS configured to handle 4GB+ of memory? Although many motherboards support 4GB or more, it make not have the feature enabled by default.
XP 32bit supports up to 4GB Like asdfbrendan said, your bios may not be set to read 4gb's of ram. and IMO 64bit operating systems are pointless, cause you probably wont be able to use half the programs you have right now.