Upgradeability for Poweredge 1900s and maximum FSB

I'm considering buying a Poweredge 1900.  I'd start with the lower end CPUs and want the maximum life for this server.  I read somewhere some of the older Poweredges had 2 types of boards for the same model and one was a slower FSB and therefore could not make maximum use of the new and faster RAM and CPUs.  Basically, depending on what you bought early on determined how far you could go with the server.

 

Also, I'll be using Redhat Linux for this server...any advice?

 

Thank in advance,

 

Norm