We won't know much till we see some benchmark information from the 8600. Right now it's too early to be able to answer your question. If you plan to SLI then do it with the 8800. Don't waiste mony on a lesser SLI configuration.
With knowledge of previous cars, 8800GTS is a better choice.
As a rule, never get two low/mid-end card, always go for the best single card you can afford.