Both "SLI ready" and "SLI certified" are garbage. That is, they are applied to some good PSUs but also to some crappy PSUs. Don't rely on them. Also, something that is SLI certified for two 8800GT cards may very well blow up with two GTX 280 cards which consume more.
I suggest you get a PC Power & Cooling 750W (it's $80 at newegg), if you're planning to have two 9800GTX cards or lower. For GTX 260 SLI or GTX 280 SLI or 9800GX2 Quad SLI get a Corsair 1000 W, for example.