Most boards will propably not have any problem. My machine runs at a mixed configuration for nearly two years. First I bought 256 MByte PC266 Apacer CL2 memory, later I added 512 MByte PC333 CL3 no-name RAM. It works perfect. The 512 MByte also runs at CL2.
The worst case I can imagine is that your machin won't boot, but I wouldn't expect any damages.