It's definitely worth seeing Naples and Pompeii, but I'm not sure you could do it justice in a day trip. I guess so, if you leave early and plan on coming back late.
You should try to visit the Archaeological Museum in Naples before going to Pompeii, and it is closed on Tuesdays, so don't go then! Pompeii is quite a big site, you could easily spend a whole day there. We went up Vesuvius in the morning and round Pompeii in the afternoon, didn't manage to see the whole site, but got a good sense of the place in the few hours we had there.
In Naples, if you have time, check out the Chapel of San Severo, it's most extraordinary and there are two of the most amazing sculptures there. "Christmas Lane" is also quite unique.