There was a thread on this topic not too long ago...
Anyway, just as a note Win XP x64 supports up to 128GiB of RAM, which is kind of a waste of the 64-bit since 128GiB means it's only using 37-bit addressing size...
If i remember the terms used to find this option in your bios, it's memory remapping rather then memory mapping as Jelle Mees mentioned but i'm sure you would notice. (Well, there might be mobos, which use the term memory mapping so look for anything along those lines.)
no, microsoft just limited it at 128GiB, hench server 2003 x64 supports over 128GiB
so its just a limit m$ sets to get people to buy 2003 hehe
btw here is a little guide to xp x64
http://download.microsoft.com/downl..._XP_Professional_x64_Edition_Right_for_Me.doc
3.25GB is max with the memory mapping thingy and that PAE on
there is some fix around that shows you 4.00GB but in reality its only 3.25 becose 32-bit cant allow more
if you dont wanna have any problems with memory amount above 2GB get a AMD64 aka x64(64-bit) windows like Windows XP Professional x64 Editon or Windows Vista x64 (you can buy oem with 64-bit only on DVD or retail with both 64-bit and 32-bit on DVD)
i have used XP x64 and Vista x64 without any problems, so if youn are gonna switch, check if you printer, scanner has 64-bit drivers
most manufacturs has 64-bit drivers now, atleast AMD, ATI, NVIDIA, Realtek, ULI, Intel, Creative
if you chose vista x64, that has most drivers even more than xp x64 and 2003
xp x64 is windows 2003 x64 with just another skin, it uses the exact same kernel, just some diffrent programs, heck they share the same service pack
PS i have run 64-bit Windows since xp x64 beta in 2003-2004 and i havnt had any problems
64-bit windows is even more stable and secure aka 32-bit viruses cant understand 64-bit binaires
and you can run 32-bit software via WOW64 emulator without any performace inpacks