Depends on what you mean !!
For the system memory each MOBO will be designed to work with a certain type of DDR memory and only that type can be placed on that MOBO - with a few exceptions for MOBOs that were designed to work with 2 different type - meaning it will have 2 different types of sockets on the MOBO and on those special MOBOs only 1 of the 2 types can be used at any time (ie. even on a MOBO with both DDR2 and DDR3 sockets on the MOBO only one or the other can be used at a time !!)
Now for the video card memory it is a different story since the video card handles all interactions with it's memory so a video card with any type of DDR memory can be used on a system with a different or same type (ie. you can use a DDR3 based video card on a system using DDR2 System RAM as long as it has the proper type socket (ie. AGP or PCI-e))