Everyone will have a different opinion on this, but it really amounts to what YOU will be happy with.
Talk to some local guys, club members, friends; see if you can go for a ridealong in their cars. Compare notes between them to come up with some basics that make YOU happy.
Remember, there's more to it than just horsepower- on the street torque is king, and is what is going to keep that silly grin on your face. To that end, a nice torquey cam and some rear gears around 3.55 will go a long way towards that end; a little looser of a torque convertor as well if you're running an automatic.
Think of it this way, I had an old C10 pickup with a tired smogger 307 and 3 on the tree and 3.70 ish gears that could break the tires loose at will, and if I threw a few engine blocks in the back would hook so hard it would snap your head back. Not exactly a horsepower machine.
So get some real-world, seat of the pants exposure and see what you like- "more is better" isn't always necessary.
And since you asked, in an A body I enjoy about 1hp-1ft.lb./c.i. to start, and gears around 3.55 for smaller engines and 3.23s for bigger ones- nice street manners and still able to defend her honor when needed. Not a world beater, but not a wallet-eater and still fun.
JMO.
