@Ron - Check zzz's answer above, he seems to know what he's talking about. I've opted for a replacement board from www.aliexpress.com, a used but certified 820-2850 i7 2.66Ghz (same as I have now). It comes directly from China, money back guarantee, 90 day warranty, etc. same as Apple. Cost me USD 480 but shipping is free, delivery in 3-5 days. It's not cheap, but it is cheaper than coughing up 2000+ for a new current model running El Crapitan.
Apparently the GPU can be replaced, as the one I'm buying has an AMD / Radeon CrossFire GPU instead of Nvidia 330, and I don't remember Apple offering that custom option back then, so I figure previous owner or "macexpert" the outfit selling it (http://www.aliexpress.com/store/208174) must have done the surgery.
As mentioned previously in a similar thread ([post|80491]) I'll report back when up and running with the new MB.