From 2CPU SMP FAQ:
"What about stepping and cache sizes?
There is a myth that in order to run multiple CPU's, they need to be the same stepping and cache size. This is not true. According to Intel, you must have the same family of processor, and that's it. In fact, according to Intel, you can even mix MHz! They say that if you clock the processors to the clock speed of the slower CPU, you'll be fine. Mixing cache sizes is also OK by Intel , as long as you're not mixing 1MB or higher cache chips. You're best off if you have the same speed and cache processors, but if you have to try an SMP system you can use whatever is available."