So if given a choice between upgrading the video card, upgrading the monitor, upgrading the processor, upgrading the motherboard, or upgrading the memory, which would you upgrade?
The least you should have is 400W. Anything above that should be fine. If you plan on an SLI mobo then you will need a higher one. Just make sure that a 450W PSU actually runs at 450W. My friend has had that problem before where his 400W PSU ran at like 380W.