Fillrate, geometry performance and GL_EXT_reme
3DMark05's fillrate measurement sub-tools are run in AFR mode with SLI, so you get to see maximum theoretical performance increases for SLI.
Eschewing green tones, I've coloured the 6600 GT's results in orange and red so they stand out. The key thing to notice is 6600 GT in SLI, AFR style, has single-texturing fillrate larger than that of a single 6800 GT and multi-texture fillrate larger than that of 6800 Ultra.
So the potential is there for good basic performance from 6600 GT in SLI using AFR.
The geometry performance of 6600 GT in AFR mode also shows it theoretically able (according to Rightmark at least) to best a single 6800 Ultra. It's possible performance like that, that makes the 6600 GT in SLI attractive in theory. The pitch for that SLI combination has always been upgraded performance higher than that of the current £300 SKU, for less than £300 in the end.
GL_EXT_reme is generally optimistic about the performance of 6600 GT in SLI, bar its pixel performance. Remember that NV43 has a quarter of the pixel output ability per clock, compared to NV40. In SFR mode, that output rate doesn't change, across an entire frame. And when it comes to anti-aliasing and higher quality filtering, NV43's half-sized memory bus width, compared to NV40, might seek to limit performance in the SLI modes.
So does the potential performance translate into great performance in our triplet of first person shooters?