Offset is the distance between the center of the rim and where the inner rim hub face meets the wheel hub. A +42 offset means that the face of the rim that meets the hub is 42mm out (towards the street) away from the center line of the rim. The amount of offset that is acceptable depends upon the width of the rim and the starting distances the OEM rim is away from suspension or brake components and the fender. For example, my OEM rims are a +30 offset. Taking the rear wheel as an example, going to a +20 offset and increasing width from 8" to 9" in the rear will effectively push your rim edge out 10mm (30 - 20) and another 1/2" or 12.7mm, for a total of 22.7mm towards the fender (street). From what I've been told, 28mm total push out towards the fender is about the max you want to go with before you'll need to roll your fender for an '04 G35 coupe, sorry don't know what it would be for the 05 sedan.

In your case, if you're going from an 8" to a 7" width, you're effectively coming back 12.7mm from the fender just due to the 1/2" difference in rim width. I don't know what the offsets are on the 2005 wheels, but if it's +30, then a +42 will effectively pull you another 12mm in from the fender for a total of 24.7mm in from the fender. (As for the distance towards suspension and brake components, you'll be moving 12mm inward, but the 12.7mm reduction in width will effectively pull you out 0.7mm as a final result - almost no difference).

So if you don't mind having a wheel that looks pushed in about 24.7mm (31/32nds of an inch) more than stock then you should be ok. Again that based on an OEM +30 offset. If your offset is more, then you wheel will be pushed in more, if it's less then it will stick out more.