I actually never realized there wasn't more to Fermi than just the 470 and 480 cards... are there no replacements for lower end GPUs?
In that case, the best you could hope for is the inflated prices to come down a bit. The GTX 260 was actually like $50 less a year ago... so maybe you have the right idea after all.
Here's a review showing benchmarks of SLI performance with 3DVision.
It seems with a SLI setup you can cancel out the 3D Vision hit, and basically get the full single card framerates. If you find a great deal on a 260 GTX now, and pair it up with another down the road, you'll be in pretty good shape... baring any sort of SLI issues.
Edit: You should also be aware you have to use 3D Vision with v-sync enabled, tearing is a no-no in 3D gaming. So you don't get 29 fps when you hit 29fps, you get 20 fps, since it's got to sync to your monitors refresh rate.