Basically the major difference between XP Home and Pro is that with Pro you can join a domain. Home only supports Workgroups.
Other diff -
XP Home - 1 physical CPU
XP Pro - 2 physical CPU's
In the case of XP treat physical CPU as physical socket on the board. (any number of CPU cores - with SP2 only)
I still prefer Win 2000.
Downside - It doesn't know the difference between physical CPU and cores, so a dual core CPU effectively means you can't use any more, or quad cores with Win 2000.
No fix is planned from MS for this.
What does it mean (minimum OS support)? -
Dual core - XP Home/Pro, Win 2000 Pro
Quad core - XP Home/Pro, Win 2000 Server
Single CPU/socket - XP Home/Pro (unlimited cores), Win 2000 Pro (single or dual core)
Dual CPU/sockets - XP Pro, Win 2000 Pro (single core only), Win 2000 Server (dual core max)
Basically for the Windows 2000 range each core counts as a CPU, XP can tell the diff between a core and CPU so your limit is number of sockets.
Oh and - Vista blows chunks.