It is a subjective choice. I try to avoid duplicate rigs due to space limits. 86Box, PIII (Win98), Athlon64 (WinXP), PhenomII X4 (Win7) fills a very long period without much overlap.
I happened to focus on rigs I never owned. I had a Pentium MMX, K6-2, Athlon, Athlon XP, Core 2 Duo, Core 2 Quad so I'm not interested in that.
Pentium III 900E,ECS P6BXT-A+,384MB,GeForce FX 5600, Voodoo 2,Yamaha SM718
Athlon 64 3400+,Gigabyte GA-K8NE,2GB,GeForce GTX 275,Audigy 2 ZS
Phenom II X4 955,Gigabyte GA-MA770-UD3,8GB,GeForce GTX 780
Vishera FX-8370,Asus 990FX,32GB,GeForce GTX 980 Ti