The maximum speed would be dependent on the media (cdr or cdrw). If the cd-r rated at 40x is a quality one, it's possible to burn it at 48x. The chances are you'll probably be successful burning it at 48x since a 40x cdr is a recent model (higher quality than older cdr's). Most recent cd burners have a burn-proof feature that allows the user to multitask while burning a cd. I assume your 48x cd burner also has this feature as it appears to a recent model. However, to be on the safe side, I wouldn't run any cpu-intensive programs while burning a cd such as playing games or running a seti (although I've played games while burning a cd & did not incur any problems). Doing other things such as browsing the net or running any office applications should not be any problems while burning.