Kev's fuel mod will fit a 2004 XTX.
The O2 controller won't (as there's no O2 sensor on pre 2007 XTs).
The list of mods available is pretty extensive - much depends on how fat your wallet is!

The good news is there's plenty you can do for very little outlay, or none at all. Click on the link in my signature and have a look through the full list of XT mods.
Make yourself a brew and get reading...