What is the intent of the upgrade? The IQ is pretty good already, so one really has to splash out the chips to upgrade. On the other hand, if it's that you don't care for the build quality, well, that's not so hard. Also, don't forget that the 18-55 is a VR lens, so in some ways it's not much behind the 17-55 in low light. (In other ways it is, of course.)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!