Car Conundrum

Greg and I are in the process of looking for another car. Most likely if we found the right car we would trade in his Passat for this new car. We're looking to get a bigger car than what we both have now (I have a Prius), since we take a lot of weekend trips and Rocky/Rocky's stuff can barely fit into my car now, so I can't imagine what Rocky + baby will be like! 

Anyway, there are a lot of questions I have about the purchase of this car, and if any of you are/know car dealing experts, please help us out! First of all, Greg is a Pennsylvania resident, but we'd probably be buying the car in California. But the car would have PA tags and registration. So...how does this affect taxes, at all? Do we pay California sales tax but PA registration fees? We had one dealer tell us that you actually pay the tax of the state that you're a resident in (so PA), but I haven't heard anyone else say that, and that doesn't really make sense to me...anyone know anything about this? 

I"m having a really hard time finding anything about this online, but I didn't know if anyone had any experience with this. Let me know if you do - thanks!