All depends on what sort of system you intend to emulate.
I run UAE on an old K6-II 500MHz with just 256MB / 16MB Voodoo.
Running a JIT 68040 emulation with Picasso96 and native AHI leaves a real 68K amiga for dead.
AGA emulation struggles a bit on this setup however, but then I have four A1200's handy...
