You've got some trouble coming.
Microsoft pulled all the XP service from their website, so you will be unable to upgrade an XP install disk to the right service pack for the games you want to play -- or to get ANY of the billion security patches needed to use XP safely. I discovered this by doing a fresh install on an XP based system, and was not really pleased. That system was converted to Linux Mint for a while, but now runs win7, so you can see the MS business model decision. If you run XP you need to get to a later service pack to run even SATA drives -- old IDE disk drives will have a nasty failure rate.
I never tried anything like this, maybe it'll work for you: http://betanews.com/2014/05/26/how-to-continue-getting-free-security-updates-for-windows-xp-until-2019/
If you just want a PC that can play old games then use WIN7 and modern HW.
If you are building a period piece and want only parts from 2007 and prior then suggest you look at getting a complete used XP system off ebay where the seller has not wiped their XP. You may also be able to get XP from somewhere else patched to the right level, however the version of XP you install must be compatible with the HW you are installing it on.
UPDATE: Nevermind, I should have known you can get anything you want on ebay including service pack 3 level XP installs.