the D700's seem to be a worthy upgrade over the D500's. The Mac Pro: So what's a D300, D500 and D700 anyway? We have answers | Architosh
after effects doesn't really use the gpu, unless you have third party plugins that do.
there have been threads on here and various other forums about problems with the D700. it doesn't seem to effect every mac pro, and im not sure if there was a resolution or not. so it may or may not be an issue you run across if you buy the D700's.