Thurgo,
I have just recently purchased a Asus SLI mobo and I looked at this Power Supply earlier from New Egg:
http://www.newegg.com/app/viewProductDesc.asp?description=17-167-010&depa=0
I am using the power supply that came in the case that I ordered and it is 480W. I am running one nVidia 6800 card right now and it is running stable even when I play online games (mostly Unreal Tournament). The link for my case is below:
http://www.newegg.com/app/ViewProductDesc.asp?description=11-105-146&depa=0
I decided to keep the 480W Power Supply to see if it will suffice for now and it is doing well so far. Please keep in mind I have not overclocked anything yet. I plan on overclocking the mobo, graphics card, and my RAM.
I will look to upgrade when I add another graphics card or if the overclocking gets to extreme for temperature. The 600w Power Supply link I pasted got great reviews but I do not think any of them are using a SLI mobo. For $50 it seems like a good deal. Others in here may be able to tell you if the link for the Power Supply that I posted would be good for you better than I could. I am just an ameteur and this was my first build completelty on my own. However, I will be happy to share any information if needed.
Good Luck
Asus A8N-SLI Deluxe : ATX Socket 939
AMD Athlon 64 3200+ Processor Socket 939
Seagate SATA HDD
MSI nVIDIA GeForce 6800 Video Card, 256MB DDR
OCZ EL Platinum Rev 2 Dual Channel 1GB(512MBx2) DDR PC-3200
Rosewill 52x32x52x16 CD-RW Black Combo Drive
A-Top XBlade Black/Silver Ultimate Gaming Machine With 450W PSU & LCD Control Panel
NEC/MITSUBISHI FE991SB-BK 19" SuperBright Diamondtron CRT Monitor