This is the gearing calculator basically everyone here uses.
http://scriptasylum.com/rc_speed/_top_speed.html
The 3905 Emaxx has the same trans as the brushless edition, so its been out for quite awhile. There really aren't any weak points in the trans, except maybe the slipper.
I have a 3905 with the FLM hybrid bulks and Hyper 7 diffs, I think its a great solution, looks nice too.
Only other thing in the drivetrain I'd look to upgrading eventually is the stock driveshafts. Over time the plastic shafts will develop slop around the u joints. When its time to replace them, its better to go with cvds. The traxxas ones are nice and tough, but theres always MIP too.