Hi,
What are you trying to achieve? My guess is that you're trying to increase the voltage range so that you'll have more sensitivity on the ADC. Using an amplifier is not the best solution since any noise will be amplified too, unless you use a low-noise opamp.
Typically the light intensity changes very slowly (unless you put it under a strobe light :D)
See http://www.vishay.com/docs/84154/appnotesensors.pdf and Intelligent Power and Sensing Technologies | onsemi for schematics and background information
If you just want to measure the light intensity for small period of the day e.g. transition from dusk till night , you don't need an amplifier at all. You just need to tune the resistor in series with the phototransistor, so that you have maximum sensitivity for those light conditions.
For instance, I also use a phototransistor in one of my circuits, I have 1.5 MOhm resistor in series and that gives me a good resolution during dusk and dawn. During the day, it's simply maxed out, during the night it's zero.
I also noticed that light intensity follows a logarithmic scale, by taking the log10 of the analogread and multiplying it by 100 (digital equivalent of the opamp). I get an almost linear response.