How should White continue after Nxb4 in the Evans Gambit? Recently I have been playing the Evans Gambit as many times as I can and for the most part, black plays 4...Bxb4, but someone played 4...Nxb4 and I wasn't sure how to continue.  After the following moves below, how should I continue with the Evans Gambit as white?

[FEN ""]
1. e4 e5
2. Nf3 Nc6
3. Bc4 Bc5
4. b4  Nxb4
5. c3  Nc6
6. d4  exd4
4...Nxb4 directly transposes to the main line with 4...Bxb4 5. c3 Bc5.  

In that line, white usually plays 6. d4 exd4 7. O-O!.  

Black cannot take the pawn on c3 because of 8. Bxf7+! Kxf7 9. Qd5+ picking off the bishop on c5 with an overwhelming advantage.

[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Nxb4 (4... Bxb4 5. c3 Bc5) 5. c3 Nc6 6. d4 exd4 7. O-O $1 dxc3 $2 8. Bxf7+ $1 Kxf7 9. Qd5+