netware upgrade

I work for a company that presently uses a netware 4.2 server as an application server serving one accounting program to users on the network locally. We are about to upgrade these server. Which version of netware would be most appropriate; netware 5.1 or 6.5? I’m thinking of upgrading to 5.1 but i’m not sure how long Novell will continue supporting 5.1 as they are already up to version 6.5.