"8800GTS 640MB models
Minimum of a 400 Watt power supply.
(minimum +12 Volt current rating of 26 amps)
An available 6 pin PCI-E power connector*"
Since you are attempting the minimum it may boot and run Windows but for gaming I expect trouble.
Ive got a gateway GM5474 with and AMD dual core 6000+ and Im pretty sure the stock power supply is 400 watts on this model. Im looking at getting a EVGA 640mb geforce 8800 gts. Will I need to get a new power supply or will 400 watts be enough?