If you already have XP, then I recommend using FPSC instead of FSPC X10.
You could always get X10 later when you are ready to upgrade your computer.
The X9 version of FPSC has had a lot of updates lately that have expanded the FPI scripting language.
So, it is a really good engine to get you started.
FPSC X10's editor has pretty much the same layout as the regular FPSC, so the transition later will be easy.
This way you can get started making cool custom games and use all of the model packs without having to do an upgrade.
However, as the other thread states...Microsoft is going to stop support for XP soon, so you need to get VISTA eventually anyway.
So, if you are ready for an upgrade then X10 is the way to go.
I have both FPSC & FPSC X10 and I wouldn't have it any other way.
Bottom line: FPSC now, FPSC X10 when you have VISTA.