.... Its safest to go with the same stepping if possible and available to help ensure compatibility, but not always necessary.
.... The MoBo prolly won't allow you to set the CPU's at different multipliers in order to equalize the Mhz of CPU's set for different default bus speeds, but since older PC's will have slower, very cheap CPU's, you may be able to simply pick up a pair of them at a higher Mhz.
.... Most home or SOHO apps. will only run about 1% faster on dual CPU's, though. To get a significant boost you will need multi-threaded, usually commercial-type, apps. and a compatible OS like WindowsNT or 2000.
.... For most home or SOHO apps., a faster or OC'd CPU will give much more performance for less $$$$$ than a dual setup. A Winchip 240Mhz may be a good, simple, cheap upgrade if the CPU's are slower than that.
............................................................................................................. -Buzz+