I was trying to collect every CPU milestone at the fastest possible speed. That is to say, for example, a Pentium 66MHz Socket 4, a Pentium 200MHz, a Pentium 233MHz MMX, Pentium II 450MHz, Pentium III 1000MHz 100MHz FSB Slot1, Pentium III 1000MHz 133MHz FSB Socket 370, etc, etc... and each had to be in a working system so you could sit down and experience the best class of each processor.
That started taking up a lot of room and so I had to get rid of a few and modify my collection rules. Now I just try to go for the fastest in each class; 1 Pentium 4 CPU, 1 Pentium III, 1 Core 2 Quad (no need for a Duo), etc. I also try to max out each system with as much RAM as possible, or that I can afford for that particular system. And if ECC/Parity is supported, I try to use that.
I do have 2 Pentium 4 Extreme Editions in my collection though. A Pentium 4 EE Socket 478 and the Pentium Extreme Edition (part of the Pentium D series but they dropped the D from the EE name) Socket 775. I bought the CPUs before the prices got ridiculous years ago, and they're staying right in my museum/collection. ☺
A (mostly accurate) listing of my computer systems: http://www.shelteringoak.com/OzzNet/