chess novice here.
evans gambit opening: game goes as follows:

1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Bxb4 5.c3 Ba5 6.d4 exd4 7.0-0 dxc3

i've seen a lot of games that continue with 8.Qb3, but what's wrong with 8.Qd5 ? I haven't come across a single game like that. discuss.

i'm thinking that whatever black does, if white follows with 9.Ng5, it's a sound offensive position.

Both moves give white the advantage according to game databases. According to my engine Qb3 is the best move. You are correct that Qb3 is the more popular move.

Black can do 8...Qe7 (with an advantage). The correct responses are 9. e5 or 9. Qd3.

9. Ng5 gives black a huge advantage (3pts according to my engine).
continued lines: 9...Nf6 Qxf7+ etc.

8.Qd5 doesn't really attack anything because it puts the Queen on a vulnerable spot, from which it will be forced to move again. Thus, tempo is wasted on a failed attack. Additionally, it does not have to be the Queen to take on f7 (instead of the bishop, backed up by Qb3) in order for White to win. Immediate checkmate threats are not necessarily the road to victory.

The idea to continue 8.Qb3! originated from Fischer's game against master Reuben Fine, when he was 20. The game continued 8...Qe7? 9.Nxc3 Nf6 10.Nd5! Nxd5 11.exd5 +-.
http://www.chessgames.com/perl/chessgame?gid=1043992

I don't think we ever got to see any famous games against 8...Qf6!, though.

Thanks for the responses guys.

After extensive variation exploring, I've found that Qb3 is indeed the best move. Black does have several defenses against 9. Ng5 (Nh6 or Ne5, which defent f7 further, making White's whole structure rather pointless, and with 8. Qd5 instead of 8. Qb3, it does indeed leave White's queen vulnerable after the black's defensive maneuvers. Thus, Qb3 is the best choice.

Furthermore, as mentioned above, Ng5 is not as effective as what Fischer did by 9. Nxc3. White takes back a pawn from the earlier gambit, and Black can either exchange his Bishop for Knight, or he could do something else like develop his other knight. Black will then be able to castle, and the f7 square can no longer be White's target.

Basically, I think you can classify attacks into five levels of judgement. (I'm just making this up btw. :P)

The first level would be, attacks in the sense that they just finished capturing or winning/taking a piece (or checkmating).

The second would be attacks that force an eventual win because, while the enemy's material is not immediately won, its loss is inevitable.

The third is an attack that the enemy can successfully defend against, but the enemy is nevertheless forced to reply defensively and give up some sort of positional tempo against the time needed to build up aggressive play of their own plans.

The fourth is an attack that, at best, momentarily causes the enemy to defend itself, but creates more weaknesses for you than it does your opponent. 8.Qd5, in this situation you present, would be such a move because after 8...Qe7 (not blocking Black's bishop on f8 because it's on a5, not f8) develops a piece on the center file and prepares to attack the Queen with ...Nf6.

The fifth we hope to never make. It immediately opens up an opportunity for your opponent to attack you, usually with enough aggression that you're forced to abandon your own.

So that would be the scale for balancing these moves in my experimental opinion. Problem is, not all moves are a pure matter of defense vs. offense; some of them are simultaneously a natural blend of both, just simple development that strengthens your position in a way that gives you attacks later.