Atm i think the only reason to go for a GTX590 is when you dont overclock and like silence.
But i dont think anyone aiming for performance like that isnt going to overclock. and also when you put the 6990 on water it would be just as silence as a gtx590 on water.
Its nice that the GTX590 runs silent but i dont think there is anyone that care's about it when spending this amount of money, you want raw power then and be able to overclock like hell.
If i had the choice i would go for:
1. SLI GTX580's
2. CF 6950's unlocked to 6970's
SLI GTX580's on top for best performance and unlocked CF 6950's second as best bang for buck