The power savings would be offset by the cost of upgrading but if want want to be more environmentally friendly theres that angle...
They will be no doubt be cooler runnner and quieter as well however. You'd be looking at 150-200 watts less at load. Obviously you'd get better performance as well.

When people are refering to throttling thats kind of misleading. Basically the gpus boost functionallity is very similar to the Turbo functionallity in Intel cpus. If there is thermal and tdp headroom, the card can clock higher, if not it will run lower but no lower than its base clock, no different than a cpu. I wouldn't call this throttling whatsoever... It's about increasing efficiency by keeping the card as close to tdp thus as busy as possible.

Given you are using 3GB buffer cards now, I'd try to get a hold of 4GB 670s given your intent to use surround.