Doesn't matter if it says SLI ready. My power supply is a 460 watt Enermax, certainly would never be certified as "SLI-ready" because it doesn't have the wattage. But it's running my 8800GTS 512s in SLI and my overclocked Core 2 perfectly fine.
If I can run 2 8800GTS 512s on 460 watts, you can run a single GTX 260 or 280 on 600 watts without a doubt.