Yes, it'll usually work fine but there is one thing you need to watch out for-the size of the new memory stick. Most 256 MB sticks nowadays are single-sided (i.e. memory chips are only on one side of the stick). Many older motherboards, such as the AX6B in my machine, can only accept up to 256 MB per memory slot, and require double-sided memory in order to reach that capacity. Single-sided memory may not work at all, or usually it will but it will only be recognized as 128 MB. Ditto for motherboards which support up to 512 MB per slot. The single-sided 256 MB sticks will work fine, as will the double-sided 512 MB ones. However, the newest single-sided 512 MB sticks will either not work, or be recognized as 256 MB. If you have the manual for your motherboard it will usually tell you the maximum supported memory capacity and the maximum per slot. Armed with that info, you can then pick memory which will work. In general faster RAM will always work at a slower speed, but the mismatch in densities is what led to the myth that PC133 won't work in a PC100 motherboard.