Ok, here's a few things to check.
-Make sure you have the latest chipset drivers from Nvidia (I think the 5.10's are the latest)
-Install the newest non-beta Nvidia VGA drivers: 66.93 or something (don't install the beta 70.xx drivers, they are causing tons of problems for people)
-What is your cooling like in the case? How hot is it getting?
-What are your CPU temperatures like?
-Is anything overclocked? If so try running at stock speed and see if it still happens.
-What kind of timings are you running that RAM at? Try loosening the timings (and/or upping the voltage) to see if that helps any.
Unless it's a faulty PSU 400W should be enough for a full system. How old is that PSU? I didn't know Antec had a (current) 400W model; I figured like most manufacturers they had a 330W/350W/430W/460W, etc.