Well, technically 2 fans are better than one. Check reviews on both cards then decide.
Yes you can take out the HD 6950 and put it in the new rig, but depends if the HD 6950 will be there or not

but usually you'll be getting a second hand one or high priced new one.
Regarding the PSU, you can grab a 700W good quality PSU from now to consider your CF in the future.
You asked so many questions and i'm going to sleep now
😛, decision is up to you.