I used my SLI for 1280x1024 res with all games settings turn up with 4xAA. Get smooth frames all the time even with Oblivion.
That would be an example where SLI actually gets a noticeable increase over 1 card. I am looking at running 1440x900 with no eye candy. SLI might get a 5-10 FPS increase for me, not worth the money though.