If you want to upgrade rather than install XP clean, you will need either the upgrade version or the full retail package. The last I knew, you did not have the option to perform an upgrade with the OEM version of XP. The OEM version is sold specifically for installation on "NEW" computer systems.
In fact, using an OEM version of Windows to upgrade an existing system is a violation of the OEM EULA.
The good news is that even with the upgrade version of XP, you can perform a clean installation. You just need the CD from the version of Windows you are upgrading from so that the installer can verify you own a previous version of Windows.
On the other side, the OEM version of XP Pro runs up to $70 (US) cheaper than the retail upgrade. Spend the extra $70 on parts for a major upgrade. At a minimum, a new motherboard CPU and RAM. That way, you can have a system that will be functional for several more years than you would with just the OS upgrade.
Chas