Windows XP Pro covers up to two processors. Having more doesn't mean that XP actually uses the left over processors. In that case, you need to go with Windows Server 2000/2003 where the different editions will allow more then 2 processors to be used. It's still just one OS license.
It's when you go with clustering that you need to realise that an active/passive system, or an active/active one, is not a single server, even if you have a virtual server name that covers the single instance. Therefore, you need to buy a license for each server you're installing... Enterprise Edition of 2003 will for example cover a single computer with up to 8 processors inside.
Microsoft changes the number of supported processors from time to time, especially for the Datacenter edition which used to support 32 processors, and then R2 editions and 64bit versions went to 64 and 128-way systems...
This is independent of having a cpu with two cores btw, for example a P4 cpu with 2 cores would still be just one cpu physically, and you could place two such CPU's in a PC with XP, and it would run just fine using both CPU's, even though the task manager would actually show 4 cpu's in use.
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