Ok, I feel obligated to say it, then gonna shut-up about it: this is a lot of money to invest in a machine when new sockets are just around the corner.
Now back to topic, most reviews I've seen set FutureMark stress test for GTX 480 in SLI at about 800W so you probably already have enough headroom. If you want even more headroom, you might want to consider using ATI cards, I think the same setup with 5870 is XFire will consume 200W less. Moreover, unless you plan on gaming at 2560x1600, such a setup is probably overkill.
Just around the corner, when would that be?
I know ATI is coming out with some new cards that might drop the price of the current selection but I hadn't heard about anything big coming from Intel.