can I mix ddr ram bus speeds
 

I have a motherboard at moment that has a maximum bus speed of 266, therefore when I put it together I used pc2100 ddr ram. I plan to add another ram module and due to price and next motherboard upgrade to have faster bus speed, I am wondering if ddr is same as sd ram was. I used to mix 100 and 133mhz sd ram in systems all the time. With regular sd ram they ran fine together, both at 100mhz.

Am I able to throw a stick of pc 2100 and a stick of pc 2700 ram in the same machine and expect them to both chug along at 266mhz without any problems?

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.
Damien