Well, how's the performance of the server (IO, network, memory stat etc. )? Are you getting any issue? I have seen many admins with "if things are working, why bother!" approach. This is good as things could be broken easily when you deal with such an old system. You may check for Virtualization solutions as well. There are P2V solutions around.
As far as the application you are running on the machine, I am not really sure of migration of that to a newer platform. Migration always becomes a pain in the neck.