I would presume that 2 7900GT's would kick the pants off of a single directx 10 card that is say the same price as one 7900GT.
That's presuming alot.
No one knows for sure, speed boosts might or might not be enough to favour one or the other.
Should I forget to go SLI and wait for the DX10 cards?
I'd say forget about it unless you NEED it now, in which case you can always sell the two cards later when the DX10 cards do start testing, and you feel the NEED to buy one then.
Either or, you're asking about intangibles that don't exist and there's no impirical evidence for either.
It's like asking which care would be more fuel efficient in 2020 a Ford or GM?