What I really love is the form factor. I had graphics card failures both on my 2010 and late 2013 12 cpu systems. When the 2013 failed I just put it in a shopping bag and took it to the Apple store. My 2010 is a b$%$% to move.
Hmm, let's see. 2013, only place to get GPU is at Apple or used "pulls" on Ebay, so it has to go to Apple store. (Used "D700/7970" is $1,000)
2006-2012 MP, remove GPU- replace GPU (lighter than the nMP, and less likely to get machine damaged) Used 7970 is about $200-400
And there are a variety of vendors selling the cards...
If you can't sell the 5,1 via shipping, put it on CL. Cash deal, no returns, no worries about shipping.
Done.