HP Compaq nx6320 T2400 @ Organiser World $1429
You wont find a lot of core duo's in this price range and this is an excellent price on it. It wont run Vista with its current config but comes with XP pro. If you really desperately want vista then just upgrade the RAM - should cost around $150 for another 512k chip. Personally - who cares whether its running vista - so long as the machine does what you need it to do.
No cashback with OW.