Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
Another thing that just came to mind, what size of a case do you have? and does it use standard sized ATX power supplies ?
Since it would seem that the Asus Essentio Series comes in a bunch of different shapes and sizes.
Also take into account that those graphics cards are up to about 10.5" long and take 2 expansion slots.
The cheapest reasonably 500W PSU is the Corsair CX500, which is ~$60 and there is a $20 MIR.
Also still today it has a 15% OFF coupon code.
Your motherboard can handle a 1500w power supply if you just wanted to blow $350 on one. The parts only pull as much power as they need and the PSU only supplies as much as the parts pull. There are no power supplies that are "too much" for any motherboards.
I listed good brands above if you want to shop around. You have not said where you would be buying from but if you are in the US and use newegg then rvilkman posted a great deal although he did not link to it. The CX series are not best PSUs ever but they are solid and can't be beat for that price.