A number of credible studies have confirmed what most of us have known for the past year:
1) XP Pro boots much faster than Vista
2) XP Pro runs applications significantly faster than Vista
3) Vista is a resource hog - the hard drive often chatters away from [i]swap file[/i] exhanges boosting memory by a trick known as [i]virtual memory[/i].
4) Vista will annoy you with [i]permission denied[/i] menus whenever you wish to reconfigure something.
SP3 is a [i]rollup[/i] of all the post-SP2 patches and [i]hotfixes[/i] issued by Microsoft, plus some additional functionality incorporated into Vista. Tests showed that XP Pro actually runs 10% faster with SP3 (compared to SP2).
So, stay with XP SP3. Unless you wish to incur a bunch of annoyances for the sake of a few glitzy showpieces ([i]i.e.[/i], the aero interface), you already have the best operating system installed. Keep it safe and well tuned.
Rick/Portland, OR