Do you need the other one? Probably not. Can you put the other card in a SLI setup? I don't know. When it first came out it was stated that the cards had to be the same, you have 2 cards by different manufacturers ableit both 7800GTs there may be differences. I'll be interested in seeing if anyone has done the SLI thing with 2 different makes of cards as I don't know if it causes problems.
Regarding whether you would notice much difference I would think you wouldn't but I've not good such a good graphics card setup as you.