First off the 6600 256 version is basically just an SLI format 6600 128 GT
I dont quite understand what you are saying here. The 128MB I can get is an SLI version.
I wont have money for a second 6600GT for a long time, but it is a possibility in the future.
If the 256MB version gives me a 25% increase over the 128MB version, it may be a better investment for me. But spending all that extra money for 5 - 10% is probably not worth it imo.