Dell Dimension 4700 running XP Professional and Mobile Hotspot Cards

I have a Dell Dimension 4700 running XPS Professional I bought in June 2005 and I have a Dell Inspiron (N5030) Notebook running Windows 7 Home Premium, 64 Bit I bought in December 2010.  I want to purchase a Mobile Hotspot connection Plan.  Are my two systems, especially the 4700, able to be used in such an environment?  Do I need to purchase something more for the Desktop?  It seems everytime I think I can do something to improve my connectivity, it backfires and I find I am unable to do it.  I live in the boonies and I presently use a Sprint 3G/4G Mobile Broadband USB Device.  But I want to use both computers at the same time and my son who visits frequently wants to be able to connect to his work network.

So what do I need?

Thanks!