Very cool tree!!!
I have run into this problem in trying to create something grand with lots of geometry only to find that my machine cannot handle the subdivisions. I have settled for trees with less branches and try to focus on style. One thing that I’d be willing to try in this scenario is once you are comfortable with the details you can get at this subdivision maybe try HD sub-d to push it further. I know that you can render HD now, so you might get lucky. Don’t forget to back up your model before you try though.
Another thing that I have learned is that if you create multiple subtools you can explore more details and sub-d’s per subtool. So, if you create the center section of the tree as the main tool and all of the branch sections as separate subtools you can get better results. If you are going for the illustrative approach then that might the way to go.
Using Clay Tubes or Clay Build up brushes with long strokes in the volume section of the tree and to create wrinkles for the separation of the branches makes a cool effect. After you can use the Slash1 brush to add depth between the Clay strokes. To texture I would Mask Cavity and paint various lighter browns, grays and reds and then invert mask to paint crevices with darker browns and maybe some greens.
Just an idea.