well are you planning on making your pc vista capable? visit the microsoft website to find out the requirements. and if it is going to be vista capable, you should just put on whichever os you like. i would suggest just using windows 2000 pro untill vista comes out, and then just install vista over it.
If you're going to go with XP get Home Edition, it's cheaper and it's pretty stable too. You could wait until vista comes out but who knows how long they're going to keep delaying it..... you could still keep 2000 pro but I'd go with XP, bu that's just my personal opinion.
This can go either way, why because many will agree with me that the OS their using now is ok for them. i do have to agree the XP is very stable but has the most vulnerablitities for spyware / malware and virus's. Many users on this site will tell you that Linux is the way to go also.
It all depends on what your going to be using the computer for is what i would base the Operating System i will be using, that's just my opinion though.