I suppose it depends on what you want to run on it, some things run best on a 8088/4mhz, or a 286/386/486... once the Pentium came out and people stopped writing clock cycle dependent code, the CPU didn't really matter.
386 is the smallest core you can have that runs modern OS with virtual memory. If you're looking for performance, by today's standards, you might as well ask for a Pentium Pro class CPU, it's not gonna make a significant difference vs a 486 in price and it will be much faster.
