user109: 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


user8860: The only point of 4...Nxb4 is to set a trap already noted in comments
to two of the answers:
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Nxb4 5. Nxe5? Qf6 6. d4? (6. O-O Qxe5 7. c3 Bd6 8. f4? Qc5+) Bxd4! 7. Qxd4 Nxc2+ 0-1

Interpolating 6 Bxf7+ doesn't help; the best White can do is bail out with 
6. O-O, but that's still not nearly enough compensation for the lost Knight.
However, if White plays the normal 5. c3 instead, Black is basically forced
to play Nc6, transposing to the Bc5 variation of the main line
4...Bxb4 5. c3 — so 4...Nxb4 is no better than 4...Bxb4,
and may be worse if you prefer 5...Ba5 in the main line, or if
White turns out to have a better move than 5. c3 (perhaps 5. 0-0,
or an immediate 5. d4 as xaisoft suggests in a comment to 
Andrew's answer).

user12: 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+


109: <p>Recently I have been playing the Evans Gambit as many times as I can and for the most part, black plays <code>4...Bxb4</code>, but someone played <code>4...Nxb4</code> and I wasn't sure how to continue.  After the following moves below, how should I continue with the <code>Evans Gambit</code> as white?</p>

<pre><code>[FEN ""]
1. e4 e5
2. Nf3 Nc6
3. Bc4 Bc5
4. b4  Nxb4
5. c3  Nc6
6. d4  exd4
</code></pre>
 8860: <p>The only point of <code>4...Nxb4</code> is to set a trap already noted in comments
to two of the answers:</p>

<pre><code>[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Nxb4 5. Nxe5? Qf6 6. d4? (6. O-O Qxe5 7. c3 Bd6 8. f4? Qc5+) Bxd4! 7. Qxd4 Nxc2+ 0-1
</code></pre>

<p>Interpolating 6 Bxf7+ doesn't help; the best White can do is bail out with 
<code>6. O-O</code>, but that's still not nearly enough compensation for the lost Knight.</p>

<p>However, if White plays the normal <code>5. c3</code> instead, Black is basically forced
to play <code>Nc6</code>, transposing to the Bc5 variation of the main line
<code>4...Bxb4 5. c3</code> — so <code>4...Nxb4</code> is no better than <code>4...Bxb4</code>,
and may be worse if you prefer <code>5...Ba5</code> in the main line, or if
White turns out to have a better move than <code>5. c3</code> (perhaps <code>5. 0-0</code>,
or an immediate <code>5. d4</code> as <strong>xaisoft</strong> suggests in a comment to 
<strong>Andrew</strong>'s answer).</p>
 12: <p><code>4...Nxb4</code> directly transposes to the main line with <code>4...Bxb4 5. c3 Bc5</code>.  </p>

<p>In that line, white usually plays <code>6. d4 exd4 7. O-O!</code>.  </p>

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

<pre><code>[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+
</code></pre>
