So I bought a Kill A Watt power meter...

G

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to see how much power my computers use...

My core i7-870 setup uses about 68w idle and about 150w under load - I imagine with my setup it would probably never exceed 200w (I've only got 1 hard drive and an ATI 4670 graphics card installed). I'm using a 700w Rosewill 80+ bronze psu...which seems like overkill after seeing the killawatt readings.

My question is: Since I seem to be using less than 20% of the PSU's output, should I swap the 700w rosewill that's currently in the system for an older 400w psu I have lying around. The older psu is a corsair that doesn't have 80+ certification.
 
May, or may not.
Normally the 80% eff would be lower total, However if that 80% is at say 50% load and you are at 21% of load the eff may be less and infact may be more than your 400 W with a lower eff rating as you would be at the 38% loaded.

Easy enough to tell since you have both PSUs. You already have the readings for the 700 W PSU. Install the 400 W corsair and get the values, use the one that gives the lowest power consumpution.