If you are unsure if you have 32 Bit or 64 Bit get the 32 Bit for sure. It will install on either the x86 or the x64. That would be the safest bet.
But remember that if you choose to upgrade XP to Vista there is NO way to undo it. So it would be advised to make sure you ahve around 20GB partition in oreder to install Vista.
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Phone: LG Optimus G Pro
Running: Stock JB from LG with Nova Launcher