Thought I'd update this as the next Potter movie isn't far. Also, Casino Royale made the Bond Franchise the highest grossing franchise ever. Here are the new numbers:
1. James Bond
Movies: 22
WW Gross: $4,495,234,026
2. Star Wars
Movies: 7
WW Gross: $4,342,748,947
3. Harry Potter
Movies: 4
WW Gross: $3,538,297,759
4. Lord of the Rings
Movies: 3
WW Gross: $2,923,933,388
5. Spider-Man
Movies: 3
WW Gross: $2,420,394,915
So Potter still has a very small chance of becoming the highest grossing franchise with OOTP (needing $957M+ worldwide), but will most likely have to wait for HBP to take the title.