Limits to size of shared buffers?

Are there any limits to the size of the shared buffers parameter (as in,
can Postgres go beyond a 2 GB size when I do something like

shared_buffers = 262144   # uses 2 GB

I had set my system shared memory to more than 2 GB, but couldn't get
this line to work (Postgres returned an error when starting up).  I
backed down to

shared_buffers = 196608   # uses 1.5 GB

and everything was happy.

--
Jeff Boes                                      vox 269.226.9550 ext 24
Database Engineer                                     fax 269.349.9076
Nexcerpt, Inc.                                 http://www.nexcerpt.com
           ...Nexcerpt... Extend your Expertise