XP Home/Media/Pro and Vista Basic/Premium/Business/Enterprise/Ultimate Will Use 4 cores or more.
The difference is Home and Media Center in XP and Basic and Premium Vista cant take 2 physical processors.
I think the question you are asking is will they actually be used up, and that depends on what you are running. Supreme Commander uses 4 cores. Video encoding can use 4 cores. Playing an old game... 1 core, browsing the internet and listening to music... less than one core...
What are you going to be doing with it?