What problem is it you're trying to solve?
I don't know if you're a fan of one brand over another, but last September I moved up to a VisionTek ATI Radeon HD 7850 and I love the thing to death. It's a great card. ATI has been a bit shaky with recent driver releases, but I have high hopes for their next release as the beta works perfectly.
I believe I'm getting all the precision Photoshop can deliver, though I don't have a 30 bit display (soon, though).
Noel- I appreciate your help with my research. Setting aside any concerns about AMD future, I have no brand loyalties when it come to hardware. I'll take a look at the VisionTek 7850. Should I assume from your post that there are NO benefits to using a GPU with double precision capabilites in photoshop or lightroom?
Let me start by saying I don't know what's inside the Mercury Graphics Engine. Certainly Adobe has not published anything stating the thing would benefit from double precision.
Double precision increases the floating point word size from where it carries 24 bits of mantissa precision to 52 bits, if I'm not mistaken. Since one each floating point number is used to represent red, green, and blue, even single precision floating point is going to give you all the accuracy of any document format.
From another perspective, your display only shows a maximum of 10 bits per channel -if you have 30 bit color. Most don't; most have 24 bit color, or 8 bits per channel of display accuracy.
I honestly don't know why the graphics card designers created the really deep formats - perhaps for specific CAD software or because scientific computing needs acceleration of very accurate calculations. I'm pretty sure Photoshop won't use it - at least not for a few versions yet.