You must have a SLI certified power supply if you ever want to use SLI. You also want a quality ATX 2.0 (24 pin) power supply even if you don't go SLI. A cheap one is just asking for trouble (like the ones that come in most cases) - that's a high powered rig that needs clean stable power. What brand and wattage is it?
Also, I've heard some problems with Kingston RAM lately....you might consider a different brand (like Corsair).
You can only use a copy of XP on one computer. If you already are using that copy, you'll need a new copy.
"It is the way of man to make monsters and it is the nature of monsters to destroy their makers."