With the 8800GT's and and overclocked quad core I'd really suggest the 620. The 520 could be running at 80% 12V loads during intense gaming sessions especially if you OC the cards. While a PSU is rated to carry a sustained load of a certain amount of wattage, it's not rated to carry it for an indefinite period of time. If you're dumping 350+ watts of heat into a case that heat is going to be absorbed by the PSU. Even with a moderate case temp of 38C the internals of the PSU can reach as high as 60C under 80% load which is not good. It's pretty simple really, for every 10C under a rated temp you keep an electronic component, you double it's service life. So if it's expected to live to be 3 years at 50C it'll live to 5 or 6 at 40C. Conversely if you run it at 60C it can live to be 2 years down to 1.5 years depending on how much you run it under load by a percentage of it's life. Say you run it folding when it's not in use and it runs 100% of the time it could really shorten the lifespan of the PSU especially if you lower the fan RPM's when it's "idle". During times of reduced circulation with a 100% load on the CPU the case temps could exceed the temps seen when gaming. Many people don't consider this fact either.
This is why running with a surplus of wattage can be to your benefit rather than running on the edge. A ratio of 100% load being 65% to 75% of the PSU's rated capacity can result in lower PSU temps, longer PSU service potential and quieter performance from the PSU cooling. Running an efficient PSU (80%+ at 20/50/100% loads) will mean that you're not wasting electricity when the PSU is run at low idle wattages.
Look at it this way, you spend $200 on a PSU and use that PSU for six full years or you spend $120 every couple of years on new PSU's due to them showing signs of aging or failure or just needing to keep up with changing hardware. Wattage requirements are not going to drop any time soon. I've seen this trend since the Prescott came out. A smaller die doesn't mean that wattage consumption is going down because they keep increasing the number of transistors on the die. If there were any truth to that myth then cooling solutions wouldn't need to keep getting more and more elaborate as time goes by. They'd be getting simpler and simpler since they'd be doing less work. I haven't seen that happen yet have you?
Just some food for thought.
M5A99FX Pro Rev 2.0
Phenom II 955 Black Edition quad core @ 3.5Ghz
2X4 Gigs G Skill F3-14900CL8-4GBXM
EVGA GTX 760
Cooler Master HAF XB EVO,
Some other stuff,
Even more stuff,
If the automobile had followed the same development cycle as the computer, a Rolls-Royce would today cost $100, get a million miles per gallon, and explode once a year, killing everyone inside. --Robert X. Cringely, InfoWorld magazine