Step-up is always based on MSRP price, and seeing as I doubt Nvidia intends to release a new GPU in the next 3 months - now or later doesn't really matter.
Your options that would be actual upgrades are 9800 GX2, GTX280 - or as you said, SLI another 9800GTX
out of all those options, the second 9800GTX or the GTX280 are the best options for performance. I'd lean towards the 9800GTX SLI setup if I were you, its proven to be a good video array and you can get a second 9800GTX for as little as 200$ while the gtx280 or 9800GX2 stepup would likely cost more than that.
As far as KO vs non-KO; just get a non KO, drop it in your second video slot and overclock it to your KO speed with Evga precision; it should run fine.
The only downside of SLI-ing gpus is that there is some visible microstutter in the g92s; if you are willing to go higher on the cost scale then the gtx280 is apparently very very consistant in terms of gameplay performance - but it will cost you obviously.
I think the best perf/dollar you are gonna get would be the 9800GTX SLI myself. I'd suggest sticking with that and if you want to buy a new nvidia gpu wait until next spring when Nvidia releases its next architecture
Bear in mind, AMD does have good options in gpus now; but from the sound of it you do seem to have an SLI motherboard, so I guess that always has to be considered.
So the 260 is not a real option, that would be an upgrade of $121. I am leaning more toward a none SLI option only because I have had some bad luck with SLI cards before (Mostly in Company of hero's) however if this is an isolated game, then that would be an option.