What is the minimum configuration for an application server?

Our application server has become very slow.
It is a HP Proliant with 4gb RAM and a 500GB hard drive, running Windows Server 2003.
It hosts our intranet software, our financial app and two other client/server applications.
I don’t know if the slow performance is due to the workload. Can someone please tell me what the minimum configuration for a server with such a workload is? Also, do I need to get separate servers for the various enterprise applications?
Thanks.