You are correct, you'll need to create a UnDP for each brand of server. Here are the two I know about:
HP : 18.104.22.168.22.214.171.124.126.96.36.199.4
Besides the different OIDs for each brand, you'll probably need to have the corresponding management software loaded on each system in order for the OID to return a value. For Dell, this would be OpenManage and for HP, I believe this would be Insight.
If you have a different brand, you'll need a utility that will allow you to walk or browse the MIB. It'll be a bit more cumbersome since you'll have to find manually search for the correct OID for the processor speed.
I hope this gets you pointed in the right direction.