That will depend on your motherboard. Some will let you, some won't. Make sure the board isn't going to try and clock to the higher frequency, you should be able to lock it to 266 from the bios, or if 266 is the max, you'll be fine. If the board is OK, the machine will just come up. If the board doesn't allow it, you'll get an orchestra of beeps. If the machine randomly reboots after a few minutes, the slower RAM doesn't like it and you should take one out, or you'll blow the RAM. It will survive a few overclocks, but not many. Also, the RAM timings might be different.