An 8800 gt can be had for $130, but gurusan is right, all the other cards are within a close enough distance that it wouldn't be worth it, even for sli.
If you want a cheap last generation card though, you could crossfire 3870's, as you can get one for like $90-100 now? or something crazy.
But you would be better off with a 4850, check out some of gurusan's results with his.
Oh and yes, the 1gb 8800 gt actually performed worse then the 512mb 8800 gt.