Thanks for the comments. Waiting for Win7 is a good idea. But I heard it will take about a year from now.

Regarding SLI: I am afraid of it. Many game magazine reviews showed that it works only with special software. In most cases games use only one kernel of the card and so you get not advantage over a non-SLI single gpu solution. But it drains more power.
And if your game supports SLI then you get some micro-delays during rendering because of the synchronization of both cards.