Input volume is only part of the equation. The trick to getting good sound is where you put your mic.
If you are recording dialogue, don't record it from across the room, otherwise, turning up the input to record a weak voice signal will result in more ambient noise. The trick is to get the mic as close to your subject as possible. You should turn your camera mic's input high enough that you are getting a strong signal from your subject.
I usually use sound from my closeup takes, where the mic is about a foot or two from the actor's mouth. Even if I go to a long shot, I can always EQ that same sound so it seems natural (getting rid of the "proximity" effect).
I always recommend a mic that isn't just built into the camera. Get yourself a Sennheiser ME66 or similar. Either use a boom pole or if you don't have help, use a mic stand with a boom arm.
Another recommendation is to record 2 level settings. Some cameras like the DVX100 or HVX200 will allow you to record 2 levels and only use 1 microphone. AWESOME FEATURE! To do this, set both Inputs to INPUT 2. Set one input to optimal (just under redline) and set the second input much lower. That way, if the actor shouts way into the red zone, you can use your backup track that isn't distorted, because it is set at a lower level. Your editing program can split these different leveled left and right tracks, ie replace the Right track with the audio from Left and vice versa.
Alternately, if you can't afford a decent mic, record as normal, so you have a source track that your actors can hear. Have them come over and rerecord their lines (ADR). The hard part is for an actor to recreate his/her energy from a room, so that source track is critical for him/her to hear and compare to. Many scenes (dialogue in a convertible, while driving, a fast boat, loud industrial noises in the background) will require you to dub, regardless. Trying to filter/fix the sound can only do so much, but ADR is clean. Always record room tone or background noise for a good 30 - 60 seconds. You can later use it on a separate track, but with total control of the volume.
If you don't have a sound booth (I built one with hinges, wood and bed foam), use a closet with carpet and lots of clothes, for absorbtion, so you don't get reflections or room echo. You can always add slight room reverb in post, if needed, and separate ambient (background noise) tracks. Do this and your projects will sound super!
If you have a low level track and no other option, try using a compressor to bring it up. I had an ultra quiet track, with the actor's only line and was able to save it this way. Fortunately, the set was very quiet, so the noise floor wasn't much of a factor.