Just a heads up... your machine doesn't even pull 400 watts, at max load.
125 watts for the cpu
190 watts for the gpu
5 watts per stick of ram (rough estimate)
Say 20 watts for the chipset (rough estimate)
Rest of the motherboard? 5 watts. Not a lot of power draw.
Hard drives... 10 watts per, max draw, give or take. Idle power half that, or less.
You're well within the wattage of your present psu, barring quality control/problems. The 'manufacturer recommendations' are based on things like rough estimates of available power, for 'joe average' psu. If you're using quality gear, your power ratings should be fine.
dropping the memory down to 800 stabilizes it, eh? Check your memory settings in the bios, make sure it's setting the voltages, etc, properly. If it doesn't, you'll have memory issues, regardless of what's 'supposed' to work.
You could indeed be having a power issue. Dirty power will cause the system to freeze, amongst other causes. How old is your antec psu?
PS: Never skimp on the psu for a gaming system. Modern GPU's are far too sensitive to dirty power. The motherboard has filters to help clean up the signalling... most gpu's have very little, if any. Clean power is a necessity. I prefer Seasonic, Antec Signature series, Corsair, Enermax (by reputaion and reviews), and there's a few others. http://www.jonnyguru.com/
is a good place to poke around for PSU reviews. My recommendation? Buy a quality unit, and plan ahead for your next couple of potential system upgrades. It should easily last that long, barring interference by the lords of electricity.