Alright well i will say use Google to find out what Service Packs are. I mean they have been around since Windows 98. Basically they are a collectionf of fixes to the OS. I am sorry but if you are building a machine you should at least know this stuff. It is very basic info.
As for Home VS Pro.
XP Home VS XP Pro - Google Search
Yes the games from here on out will use Vista. So if you are a big gamer you will HAVE to switch to Vista at some point.
I suggest Vista cause i dont think it is that bad. Others will disagree with me.
Sorry about the harsh words but XP has been out for almost 7 years and not knowing the difference after all this time. Not knowing what a Service Pack is. Kinda hard to build a PC and not know some of the very basics of the Operating Systems is hard to grasp.
But at least the info is there on Google to find. Dont always have to wait for a answer from someone here. Goggle will be faster.