6990 is faster, and a better designed card overall.
nVidia tried too hard to match the price of the 6990, so they skimped hard on the voltage regulators on the 590. Consequently, they locked the voltage down in the drivers, and forced ridiculous OCP, so overclocking is pretty much pointless. It is a terrible card, probably one of the worst since the FX series:
I say go for the 6990.
or if you want to go nvidia, 2 570's in SLI would be an alternative.[/quote]
Spot on. As much as I love nVidia... the 590 tried too hard and flopped :(
I personally, have had the worst experience with AMD's dual GPU cards, the 4870x2 and the 5970. After both of those fiascoes I just quit, went 580 and love it.
AMD's drivers are just so much more finnicky in CFx setups, even if it's one card with two GPU's. Funny thing is, if it works perfectly for you.. the scaling is extremely impressive in the 6XXX series. Not to say the 5XX nVidia series scales poorly, but AMD really kicked it up a notch.
Panzer, if you much have a dual GPU card, take the 6990.
If you want to go with a single card setup, it's a fight between the 580 and the 6970, in which case considering the 6970's cheap blower cooler, I'd take the 580.
If you're open to SLi / CFx configurations, SLi 570's or CFx 6950's are your best option.
Absolute best performance / price would be the dual 6950's... though me personally I'd still take the 580... I'm just a nVidia fan.. and quite frankly the fact that the 580 can stay cool while not going up to space-shuttle-launch-noise... it wins. I like my case quiet :)