Having CPU's from the same batch is the best. Most newer CPU's run fine together so long as both are SMP capable.
I have heard of someone running a 333 and a 300 together for a while and not haveing any serious issues, but wouldn't recommend it.
PS, your CPU's should be fine together. I ran a pair of P3-450's that were not identical stepping, for about 8 months (until I upgraded to 700's)without any problems.