Basically, the comment above speaks volumes.
Extrapolating the conclusion:
-> AMD 6990 performance is relatively consistent as expected across all games-> suggesting they haven't spent much time optimising drivers for particular games ... ergo baseband performance will scale with new games.
-> Nvidia GTX590 is inconsistent across games -> Suggesting that NVidia have spent much time optimising drivers for particular games .. ergo performance for new games will be hit an miss.
I'm not arguing in favour of either - you can place your future game performance in the hands of a manufacturer with better performance consistency (AMD) or a manufacturer with better ongoing (driver tweaking) support (nvidia).
I'd prefer a slightly lower, but consistent, baseline performance, rather than risk random performance fluctuations between games, and worse, on the same games, but between driver revisions.
But then, I'm not a gambler.
But seriously ... this is a niche product, that would be sheer folly to pay for, and I'm totally apathetic to the whole issue regardless, not least because of this.