Question: Can my SB600 flash

For wireless flash you need two things:

1) Something to act as the controller.

2) A remote flash.

While your SB600 will work fine as a remote flash, the D40x does not have the 'built-in' capability to act as the controller (as some of the more expensive models do). Unfortunately, you would need a SU800 or SB800 to act as controller.

Matt