I think the real question should be what power to weight ratio is needed for the street.
e36 needs 300hp
e46 needs 400hp
e90 needs 450+

I really like street cars that can take a bit of a beating (good brakes, good balance, limited slip, rwd, etc)

 

What I've driven that impressed me:

700 hp Nissan 300ZX twinturbo: My friend built this car back in '03. It had a stroker motor with fairly responsive turbos. Car would make full boost at 3,800 rpm and fuel cut was set at 8,000. The chassis was setup very well with polyurethane bushings throughout, larger swaybars, fully adjustable coilovers, competition alignment and corner balance. Great fun on the street and could put the power down properly if you knew how to modulate the throttle. But, IMHO it had about 150hp more than it really needed.

380hp Porsche 993 Carrera (3.8 EVO motor): Excellent balance. Very responsive and free-revving, but you could feel the car come "on cam" at 5,000 rpm. IMHO could have used another 30 hp or been re-geared to take better advantage of the upper end of the revs.

Porsche 996 GT3 RUF 410hp: Drove just like a stock GT3, but had that little extra oomph past 6,500 rpm. Torque was greatly improved in the mid-range. Car had supplemental suspension upgrades. A *****cat on the street that could dust off cars with much more hp when things got twisty.

Porsche 911SC 250hp: My personal car. It has a short-stroke 3.3 (stock crank + rods, but with 100mm pistons and cylinders). I kept the stock cams and it's excellent on the street with strong torque from 3,000 rpm all the way up until 6,500 where the power flattens out. With the fairly short-gearing of the stock 915 transmission it is at the upper limit of a daily driver 911SC. Any more hp and I'd be overdriving the stock suspension and brakes. IMHO it is the perfect combo for this era of 911.

I think the golden combo for a sports car is 450 to 500hp naturally aspirated or with very responsive turbos, 3000 to 3500 lbs. wet.

 

My friend has a 700hp Mit. Evo7 for drag and street use.But they are stupid to drive on the street...450-600bhp is ideal IMO...

 

The x6m i have has 550hp.. I have never been so completely underwhelmed by power, despite it being the most powerful car I have owned. my 430hp 993gt2, completely overwhelms me.

depends on the car, suspension, weight etc etc etc...

there is a perfect power for every car.

 

So its not faster than your GT2 in a straight line?

 

My penis is 1/4", so I voted for 900+ horsepower.

 

Depends entirely on available traction, weight, induction, displacement, etc, etc.

500 hp in a ~2,000 pound car is really quite fast. 500 hp in a ~3,500 pound car is "quick" but would leave me wanting for more.

800 hp in a RWD, front engine car is a traction nightmare. 800 hp in an AWD mid engine car is all sorts of fun, assuming the gearbox doesn't turn into shrapnel.

... and so on. For a FI, RWD daily driver with a relatively small displacement, ~600 hp is where I'm happy.

 

So you drive a viper is what your saying.

 

i enjoy driving the 200whp S2000 around the street. more than good enough with only 2800lbs and exceptional handling the car provides. but that's just me though

 

I am an HP junkie but 500 seems adequate