So the 590 doesn't sound good, due to its problems with overclocking. I have heard lots of support for SLI 570s, that they are equivalent to and better than a 590 when overclocked. Also, SLI 580s, but that's insanely expensive (about $1,250). How would a 6990 stack up to SLI'd 570s (or 580s)?The problem is that the GTX 590 power subsystem is really poor in reference versions, and is barely able to overclock well. The 590 uses two GTX 580 cores, but are severely downclocked. And the 590 is barely able to overclock to 580 levels, which makes it slightly worse than SLI 580's.
The 6990 on the other hand has a good power delivery system. It uses two 6970 cores, downclocked like the 590, but is able to overclock to 6970 levels. And it will overclock to about 90% or so of 6970 levels, and usually it's more thermal limited. Put it on water, and it'll probably overclock like a regular 6970. That's what makes the 6990 better than the 590, even though the 6970 core is only approximately equivalent to the 570 core.
The 570 reference PCB is identical to the 580 PCB, and in fact identical to the 480 PCB. That means blocks designed for the 580 will fit 570 and 480 cards. However, you still run into the RAM problem, 570's only have 1.2gb.
Hmm, the only thing I could find for a "non-reference HD" block was [this], and I haven't heard good things about EK.Those blocks probably will not fit the 570 HD. The HD is a non-reference design, so where things are laid out are probably different. There's a chance that it will fit, but there's also a good chance it won't.