In theory, 8800GTX sli might be better, but I think I would go with the single fast card. Some games do not scale well with multiple gpu's. The larger memory on the gtx280/285 will serve you well at 1920.
I cant find any direct comparison benchmark between 8800GTX SLI vs 280/285 but from what I can gather, it seems that the 2 options perform roughly the same (285 might have a slighly higher min fps).
While two 8800GTXs will produce relatively more noise/heat, but I only need to spend $100 to reach 8800GTX SLI vs having to spend $275 to get a GTX285 (current retail price of GTX285 is about $375 and minus the $100 that I will get by selling my 8800GTX).
Since both options perform just about the same and option A (8800GTX SLI) is cheaper, I should get a 2nd 8800GTX. Make sense?
For $100, if you can find an 8800GTX, that would be a nice upgrade. As far as being current, (directX versions, mobo interface, DDR3, DDR5, etc.) you'd think they would be good for a good while yet. That's the only thing I would consider.
I don't know what platform you are running, but saying you wanted to upgrade that sometime in the not too distant future, would the 8800's move into that platform upgrade effectively, or would you be better off moving one 285, and adding another? At that point, would you be stepping beyond a 285? Why am I asking so many questions?