What PSU would I need to go with this?

I'm building a new pc, currently housing an AMD X4 640, 8gb ram, HD 6850/6870 (depends on price of PSU) and then the usual bits and bobs.

The support guy told me that 750w would be best for overall stability, but using the Antec PSU calculator it says I'm around 350w required so even a 450w PSU would be ok.

There is quite a price difference between a 750/600/450w PSU so I'm asking for a second opinion :D