Effective stack 80% max buy-in. Villain (UTG) bets 3xBB, Hero (Big Blind) calls with 5d 4d. Flop 6d 7d Ac.Action: Hero checks, Villain bets 3/4 pot. Hero goes all-in. Is it all-in good move considering that in most cases Vilain will throw best hand and if he does't throw hero will win in more then 50% cases at showdown. Also villain will throw his hand if he's on flush or straight draw.
Given the fact that you said "effective stack 80 % max buy-in", I'll assume this is a cash game we're talking about, not a tournament.
If villain has about the same stack as you, I think the all-in move is not a mistake. Even if he has an Ace or if he hits a set on the flop, you say he will still fold. In the worst case (a.k.a if he calls and wins), you will still be winning in the long run.
Shoving with a straight+flush draw is a pretty standard play for NLHE in my book. You typically have enough equity to be even money against your opponent's calling range. That means you could call their shove and see a profit due to the dead money in the pot.
Almost any time that you could call and be even money, a shove is better, and the more money in the pot at the time of your shove, the better it is. You profit from every time your opponent folds in these spots - much more than you do if you both stay in the hand.
I made a mock hand-history to illustrate the situation.
$1 NL (6 max) - Holdem - 6 players
Pre Flop: Hero on BB has 5⋄ 4⋄
Flop: ($7, 2 players) 7⋄ 6⋄ A♣
Assuming all your outs are good and clean, then you have 9 diamonds (2⋄3⋄, 8⋄9⋄T⋄J⋄Q⋄K⋄A⋄) and 6 outs to a straight (3♣3♥3♠, 8♣8♥8♠).
Depending on your opponents range this very often equates to over 50% equity. Thus, you're likely to be the favourite to win the hand at this stage. All-in is OK but it may not be best!
There are a few things to consider when making a decision about the best play. Especially:
As I've laid out in the mock hand-history, you'd be shoving/paying $77 to win $89.25 total (your bet of $77 + current pot of $12.25).
I've tried to consider the range that your opponent would likely call with. I came up with
Obviously, your opponents UTG range is wider than this and he will fold a significant amount of the time to make a shove profitable, in addition to your strong equity.
I suppose it comes down to this, considering the other hands in your opponents range, Is it more profitable to give the opponent an opportunity to bluff some hands he might otherwise fold, by hero raising a smaller amount? I'd say yes. The only way to make this judgement is to know villains UTG range more clearly, which you don't mention. Calculating X and Y depend on this analysis of possible hand combinations.
It's difficult to come to a clear best answer but, when is it anytime in poker! I hope this gives you some ideas on making better decisions. :)
If it is your strategy to be playing these marginal hands in the first place. you probably need to have a more subtle strategy for letting their value come forth when you hit a gin draw like this. betting more of your stack than would be mathematically valid for a flush draw, even with the guarantee that everyone calls you all in when you make it, puts other people off of their draws as well. Betting about a third of your stack still will have many of the hands that originally called you all in, raising you all in, but you just call those anyway, really what you're looking to do is give the people who have your draws crushed and would have called you all in as many opportunities to put dead money into the pot and fold on the turn as often as possible. The hand that comes to mind specifically is Adxd.
This is just my intuition for this hand