What is My PE2600 Processor

While I like Dell support such as My Systems I have always had a problem with the descriptions in "Original System Information".

I have a PE 2600 with Windows Server 2003 at home.  It has a single Xeon processor and I am considering going dual.  I already have the VRM but am perplexed at what kind of processor I need.

When I lookup my System Information for my service tag it says: 

U0621 Processor, 80532K, 2.4GHz, 512K 533, DECISION ONE

To confuse matters worse the manual says it has a 400Mhz front side bus.  Doesn't this indicate a 2.4GHz with a 533MHz front side bus?

What kind of processor is it?

And is it socket 604 as that seems to be what is available on EBay? Variety of 604 Xeon as it appears there are differnet types?

Thanking you in advance,

Peter