I'm comfortable to say that you won't notice a difference in the mentioned applications, except Oblivion which I have no clue because I haven't played it. In other games, level loading will be somewhat faster but that's about it. If anything, unless you go with an NV motherboard, losing SLI will affect the performance in a very honest way.

As a matter of fact, I still maintain my opinion that a fast single-core is more than enough for current games.
Then again the upcoming NF680i looks very promising and if you like to stay on the cutting edge, you know what you want and most of the time it's hard to resist. (We all know, don't we?) You don't need others' approval for your satisfaction. Just buy what you want and enjoy what the technology has to offer! (Although X6800 seems like a lot of money for that)