The difference is easily noticeable by just about anyone. Even in 2d, the colors are a bit better due to the new 10bit ramdac. In 3d the difference is much more prominent, all of the shimmering problems with the 7x series are gone, and the AA is a lot better too.
I don't own an 8800 (yet), so I felt it best not to reply to JVC's "I don't see it" comment, so thanks guys for chiming in with what I thought and hoped was the reality. But his statement that he owns a GTS raises the issue of why he's even asking the question posed by this thread. Is he contemplating buying a set of 7950GX2's from a buddy or something? If so, I'd say a big no. The power draw is horrible, the effectiveness of Quad-SLI is spotty at best, and the tech is now outdated. I'm sure that for less than the cost of even a used pair of GX2's he could buy a second GTS for SLI, a setup that would absolutely crush the GX2's.