I have an old Pentium M notebook with 256 RAM lying around with Linux for optimizing. If you get it to run fast in it, it should run pretty much anywhere. I'm looking to moving to an old netbook though, but I'm not sure because of the architecture of the first Atoms.
If I was optimizing for ARM the RPi would be a great machine :)
The Penitum M is sort of like "the missing link" between old and newer architectures. From it "evolved" the Core Duo brand (I also have one of those around), Core 2 Duo, and Core i3/5/7 architectures. And while there are differences, the M is still similar enough to be representative.