I will most likely put XP on this system and save the old system for linux. Or even dual boot.
I haven't got vista, so, I don't know how compatible it is with current games and/or compatible with older games.
As for video cards, I thought dual cards work faster than a single? Must be false advertising getting into my head :)
The PSU was just randomly picked since the price for that 600W is the same as 500W.