haydenaurion: I hate to be a pain, but what GOG versions of games have you tried on it?
You're not being a pain in any way, i'll be glad to help you with your decision.
So far the GOG games i have played on my current GTX680 were:
Darkstone, Disciples, Ultima VII, Syndicate, Giants Citizen Kabuto, Septerra Core, Iron Storm, and Blood 2. I think that's it, if i remember anything else i'll post here.
I also have a few old physical games here that i ran recently on my GTX680, such as KKND2, Chasm The Rift, Meat Puppet (i had to make a VM for this one though, since it does not work on any OS above Win 98) and Dark Earth.
Honestly, when it comes to old games the OS is a bigger problem than the GPU. I still couldn't find any game that didn't work on my PC.
If you are so concerned about old games i really suggest that you keep a XP partition on your HD, because some old games don't even install on Windows 7.
The GTX680 is an expensive card, buy if you really want performance, you can't go wrong with it. It's the best single-GPU card on the market right now. And it's compability with older games is not different to any GPU released in the last 6 years (when the current shaders architecture became the norm).