I have much the same setup. (MSI K8N Diamond Plus, email@example.comGHz, 2x7900GT XO's, 2x1GB G-Skill, 74GB Raptor, Additional HDD's and Optical Drives and so on) The two 7900GTKO's I'm running are clocked at 570MHz/1720MHz (they overclock well). They will out run a single 7900GTX in almost everything. It's going to cost at least $200 to step up to a single 7900GTX, I say forget about stepping up, just order up a second 7900GT KO, they are $275 on Newegg, that's only $75 more then the 7900GTX and it will preform proportionally better. It would be a waste of money to step up to a 7900KO Superclock that would cost you about $50 for the stepup and $320 for the card making it like $100 ($370 total) more to run two 7900GT KO Superclocks then two regular 7900GT KO's. The way I see it you have two options that are most cost effective, either 2x7900GT KO's (which I reccommend) or a single 7900GTX.
(I am assuming you paid around $300 for the 7900GT KO)