Orly to De Gaulle - Paris

depends on what carrier you use at cdg, it's a big difference! cdg is very big and some carriers' boarding gates (like NW) are put to very far, not to mention the security checks... so if you have to check in your stuff in CDG then board it with NW, 3 hours is not enough.

I faced the same problem changing airports in London, en route to Spain. In my case, I was getting into Heathrow at 8:00 AM with an 11:50 flight to Spain from Stansted. Even if it had been right next door to Heathrow the time would have been tight to get checked in and through security in time. Turned out it's 90 minutes by bus from one airport to the other. I'd suggest taking the flight the night before just to be safe.

Not a chance. It took us 2 hours from the moment we got off our plane in Orly until we arrived at CDG, via Orlyval and RER....so you won't be at CDG early enough to check in for your flight home. Security at U-S airlines in Paris is extraordinarily tight, requiring more time than usual to get thru. So spend the night in Paris. You can book an inexpensive hotel out at CDG (www.accorhotels.com, and look for an Ibis or Etap hotel at Paris Nord 2) for that night, then have a comfortable sleep-in before taking the shuttle to the terminal.