Did I underestimate my PSU needs for SLI?

[geno]

Using the various PSU calculators around the web, I figured I didn't need an insane PSU to handle a decent SLI + Quad-core setup, but I'm starting to have second thoughts.

My intended setup is the following :

Intel Q9300 @ 3.0Ghz (maybe a few more hundred Mhz, we'll see)
Xigmatek S1283 cooler w/ 120mm fan
EVGA 750i FTW motherboard
2 BFG GTX 280 cards
Creative X-Fi PCI-E card
2 Sticks Mushkin DDR2 @ 2.1v
1 WD 500GB HD, 7200 RPM
2 DVD-RW drives
5 80mm fans

Now, every calculator I found for this setup said I'd be under 600w at full load, so I figured a 750w unit would do just fine. I ended up picking up a Xigmatek 750w PSU with 4 12v rails @ 18A apiece. Link to specs

I see many benchmarks / reviews showing actual power consumption under load, and I have yet to see many systems dig into the high 500's. Most register in the low 500's with a similar configuration. Pretty much everything I've seen falls below 80% of the PSU's total wattage. However, on SLIZone.com, I'm seeing that the minimum recommendation for SLI'd 280's is an Antec 850W unit, with a slew of 900w+ units listed. Can I make it by with what I've got or should I have gone bigger and better?

 

[theAnimal]

You'll be fine.

 

[Zap]

I think that Xigmatek does 60A, which should be fine.

NVIDIA seems to have minimum wattage levels for "certified" SLI PSUs. They aim for overkill a bit to minimize potential problems.