600, 700, and 800 Series

These routers fall into the category of SOHO (Small Office-Home Office) and
should only be used to augment a home lab. They can be useful for practicing
dial technologies such as ISDN and DSL, and they're inexpensive. Unfortunately,
SOHO routers run a limited IOS release, (the 700s are non-IOS) and most models
aren't suitable for back-to-back connection.

1000, 1600 Series

These run IOS; you can get serial/Ethernet models; but you won't find on these
several advanced routing protocols and legacy desktop protocols you might find
on an exam.

1700 Series

This new series has a lot of available security and encryption extensions to
IOS, but are limited on routing and desktop protocol support.

2500 Series

These are the first of the "enterprise" routers in the current Cisco line. You
can run the latest full IOS on these, including virtually all the available
features and routing protocols. They're around $1,800 new depending on the
model you select and can be upgraded with up to 16M of flash memory and 16M or
18M of RAM. The most popular certification box, the 2501, supports two serial
ports (DB60), Ethernet, and DCE clocking. Some of the latest security features
on the IOS firewall are starting to require a faster processor than the 68030
in the 2500.

2600, 3600 and UP

Modular routers intended as an eventual replacement to the 2500 and 4000 series
routers, they may be overkill for a certification lab, but you get a lot of nice
features: full IOS support, quiet fans, lots of memory expansion, and fast
processors.