I also have two GTX 285s in SLI (aren't they great?), and 'tho FSX may not benefit from SLI, I've found my rig can handle AI traffic at medium-high while keeping acceptable frame rates. My best suggestion is to experiment until you find your optimum settings.

Also, consider getting the add-on utility, 'TweakFPS for FSX' at TweakFS.com . They have a 30-day free demo so you can try-before-buy.