I can't comment on the overclockability. I have heard that Oblivion won't use more than 256MB of video ram, but that is just a passing comment on a forum. You should do more digging. Most exisiting PC games are tuned to work well on 256MB or less of video ram. The CAD may or may not do better with 512MB of video ram.
I was hoping someone else could jump in, since these are just rules of thumb that I'd use. I would buy an nvidia 6600GT right now if it was my money. Hmmm.... actually, I already have one.
