PA has a lot of wierd rules and regulations. Your best bet is to go to your local Notary, explain the situation to them, and let them tell you exactly what you need to do. They may start the paperwork for you, or give you paperwork that the seller, or Illinois DMV needs to fill out. They may even ask for the car information, and issue you some type of temp tag. It'll take 10 minutes, and save you months worth of grief if the title gets bogged down in incorrect paperwork. Might even save you a ticket.
Also, give you insurance agent a call. If you try to put plates on the car, you will need proof of insurance. I believe you can use any current insurance card, and it will provide you with 10 days of coverage. But that's how State Farm does it, and I dont know if that is a universal thing, or if it differs according to company or policy type. You know what they say about an ounce of prevention...