I think you will be fine getting the 20" .325 bar and chisel chain combo, if I were you I would also consider a simple muffler port-mod. Basically opening up your existing muffler port (or adding an additional port) to your muffler. Makes a sizeable difference in how the saw breathes, well worth the work doing it. Make sure you do it correctly, in the end you want the new muffler exit port to be just a tad smaller than the outlet port on the cylinder (where the muffler bolts onto the cylinder). You WILL have to re-tune the saw, this really makes the saw breathe WAY better, but it will be louder too. Kinda like waking up some extra HP out of your saw. I muffler-port all of my saws, it makes that big of a difference......
I will be doing this, I think I found a thread on another site with a muffler mod. Now if I can just get all the wood split and stacked in my yard I'll have time to do it.