Im getting ready to flip back from Vista Basic to XP and Im adding some memory and one or two other upgrades and was wondering if it's a better Idea to install the upgrades before I change the Operating Systems? Or can I install the upgrades and then just wipe the slate clean as Im doing the actual change?

What are the other upgrades?

If you are going to remove Vista and then install XP Clean then just change the hardware and install XP.

If you intend to retain Vista as a dual-boot then change the hardware under Vista first.

Be aware that Vista via its' EULA states that only one major change of hardware will be accepted per Vista version. Hence if you have any plans to use your Vista license on another computer do not upgrade the hardware under Vista or you will not be able to use it again.