6950 x2 is better. IIRC, the 6990 is somewhat less powerful than 6950 x2, and I think the noise and heat would be worse from the 6990. The HIS 6950 you listed is a "turbo" edition slightly different than the standard ref card in the sense that it is clocked at 840 mhz instead of 800, though I don't know if that would change its unlocking potential. I'm trying to answer that question myself too so if anyone can confirm that it is unlockable, I would greatly appreciated it. The XFXs are 1GB and therefore can't unlock.

Don't get the HIS 6950 2GB, they have high number of defective cards. Having issues with Fan speed control.
Basically the fan will blow at 100% all the time, no matter what you set it to, as it doesn't take any commands from the driver at all.

1 6990 under water will be better than two 6950s under water as the gpu's used for the 6990 were binned better.

as for the 5850s, a lot of the non ref ones are the cheaper ones. google around to make sure everythings working fine