SLI is strictly for 3D (i.e. gaming or 3D software) SLI helps a lot when using 3D CAD software, I use softplan and there is a noticeable difference when rendering the 3D images, but while running Photoshop, the Nvidia 4800 or 3800 are great cards and they are even great cards for the 3D software.
Thanks for the input gentlemen.
I did find additional information from the NVidia SLI forum; bottom line -- no effect or advantage for CS4 Photoshop rendering.
CAD software and a few games which are specifically coded for using SLI do indeed benefit.
I've decided on one NVidia GTX 295 for my new system; with 1792 MB of GDDR3 memory it's probably overkill, but it will presumanbly keep me happy for a couple of years.