(1) Yum yum.
(2) No comparison; the 2600 has a real CPU. Filtering oomph, BGP,...
(3) 4 t1s easily, though cost-wise you probably only want to put 2 on it
(getting a 2nd router for the next two WIC-t1 ports costs $2k vs. a
$1500 network module to expand the first 2600). Having yet another
router is probably worth another $500.
(4) A good app for the 2-port ethernet version is doing full-duplex ethernet.