Database upgrading: upgrade server first or client first?

I'm running a postgresql 8.1 database on a single server which is queried from a number of client systems.
Both server and clients are running Debian Linux.

I need to upgrade from 8.1 to 8.3, but it's difficult for me to upgrade everything at once. Is it possible to
run different versions of the client and server, and if so is it better to upgrade the client first or the server first?

When I say "the client", I mean the psql client and the libpq library (I actually use Perl DBI, but I think that ends
up calling functions in libpq).

My gut feeling is that later versions of the client should detect and work with earlier versions of the server, so
I should upgrade the clients first. But I can't see anything definitive on this.

Any advice or pointers to FAQ items that I've missed would be appreciated.