The water heater is to maintain a consistent temperature in the water so it's not going up and down every day and night (constant temperature fluctuation can cause undue stress for captive reptiles which in turn makes them more susceptible to disease). Maintain a water temperature around 75 degrees as xsavingsaturdayx (you need a shorter name!

) said.
The light bulb is for warming up the basking area and to make the turtle feel good

. The turtle needs to be able to climb all the way out of the water to dry off and warm up. This helps prevent shell rot and aids in digestion. You can use a regular household light bulb for this. Don't bother with the expensive "daylight" bulbs at the petstore as I have found that they don't last long at all. The key is to maintain a basking temp of 88-90 degrees.
So, you don't need to bother spending the extra bucks on that useless UVA bulb...BUT you DO need a special UVB bulb. This is crucial for the turtle to metabolize calcium for their shell and skeleton. The UVB bulb comes in the form of a flourescent tube (requires a strip light fixture), a compact flourescent bulb (screws into a regular clamp lamp), or a mercury vapor bulb (more expensive but provides both heat, uva and uvb). If you get one of the flourescent bulbs make sure to get one that's labeled 5.0. If you go mvb, get 100w flood, use a ceramic base lamp and place more than 12" away from turtle (these bulbs generate heat and uv very well, you don't want to burn your turtle). Make sure to use a thermometer to test the basking temp.