PSU for 2 x 9800 GX2 in SLI

You need 2x8pin and 2x6pin connectors for 2 9800GX2s.
That sums up to 500W for the graphics cards (theoretical upper limit).
I'd recommend you go for a very high quality >800W unit.
Corsair HX 1000W ($280)
Enermax Galaxy 1000W ($240 after MIR from newegg).
Or something in the same 800W-1000W range and Tier1 to Tier2, from here :
http://www.xtremesystems.org/forum [...] ostcount=2

That being said, wait until the end of June(18th or so) for GTX 280 to come out and buy a graphics card then.

third vote on the 'waiting for a month' strategy.
If you are planning to spend that much on a new rig, it is not good idea to rush things only to find out month later that you could have saved a load of $$$.
GTX280 in SLI *might* be pretty close to 9800GX2 in quadSLI performance wise and total power consumption should be somewhat lower....
The new cards wont necesseraly be that much faster, but it is likely the prices will come down on the older parts anyways.

Wait a month.. 9800GX2 is a stopgap card. SLI for that would be a HUGE waste of money. Don't fall victim.

SLI "down the road" is also a horrible plan. If you like to spend more and run up the power bill for lower performing cards its an incredible option though.