Indeed, I think White's best after 4...exd4 5.c3 Na5 is to persist in sacrificing a pawn for compensation after 6.Bd3.  It may not be enough for a theoretical advantage but I rather like White's practical chances and the compensation should be at least sufficient.   

5...dxc3 is dubious because as well as the 6.Qd5 line (after 6...Nh6 7.Bxh6 0-0, both 8.Bxg7 and 8.Nxc3 are slightly better for White), White also gets the upper hand after 6.Nxc3 because if 6...d6 (otherwise 7.e5 follows) 7.Qb3 leaves Black unable to defend f7 and 7...Na5 8.Bxf7+ Kf8 9.Qa4 is slightly better for White as Black's bishop-pair does not fully compensate for the misplaced king and lagging development.

In addition I believe that White can get a small advantage due to the better development/central control with the simple 5.Nxd4.

JediKnight has analyzed the line here:

http://tws27.50webs.com/chess/scotch_gambit/minormoves.htm

kylemeister wrote on 07/17/12 at 14:20:31:


5. c3 can be dangerous , tempting 5. .. dxc3 6. Qd5 Nh6 7. Bxh6 0-0. This doesn't win a piece without compensation, since if 8. Bc1, then 8. .. Nb4 gives dangerous counter-play. Returning the piece with 8. Bxg7 is likely to be the best try.

But there's an alternative with 5. .. Na5 . White then has the choice of whether to concede the Bishop pair with 6. Qxd4 or a pawn with 6. Be2/d3 dxc3. 

(There was some analysis of this in the quarterly NIC Yearbook)

ghenghisclown wrote on 07/17/12 at 07:57:23:

Underrated. 

I believe that the white square bishop does not belong on g4 in the Hungarian defense.

I find that it's most efficient job is to put direct pressure on e4 from c6 or b7 and then to eventually transfer to the Kingside (f5 or g6) to later pressure the central light squares and sometimes invade the enemy camp on d3 or c2.. Trading it for the Nf3 is usually a strategic mistake.

An under rated defense at mortal levels. (below 2400)

The aljechin-game is:
4.c3,Bg4 5.Qb3,Qd7?! 6.Ng5,Nh6 7.Nxf7,Nxf7 8.Bxf7,Qxf7
9.Qxb7,Kd7!! 10.Qxa8,Qc4 Rodzynsky-Aljechin 1913
That's why its sometimes referred to as the Aljechin-variation

book recommendation is (usually with c3 before d4)
6 Be3 Nf6 7 Qb3 Nd8 8 Nbd2 g6 10 dxe5 with the idea a4-a5.
I had this variation once on the board (that was what I was looking up) he played 6 Bxf3 gxf3 (probably queen is better) 7 Nf6 Qb3 and black was defending for a lot of moves, though he got a draw.