The nice thing about DDR is that it is largely backwards compatable, so if you get ram that is above spec then the mobo will dumb the ram down to run at what it can handle, and if you get ram too slow, then the mobo will slow down the match.
Likewise, if there is ram already in the system then all ram will run at the speed of the slowest stick of memory. So long as it is DDR1 in a DDR1 mobo then it should work.
All that said; typically people put the fastest ram available to their mobo at the time of building, and the general rule of thumb is that you want to match what is already there.
If you check the mobo manufacturer's website it should have a spec sheet with your real answer.