The Intel 945 chipset supports both dual channel and single channel (asymmetric) memory modes. To use dual channel, which is by far the preferred mode, you need two memory sticks of the same size. If one of the sticks has a slower speed than the the other, the slower speed is what will be used.
If you use only one stick, or two sticks of different capacity, like a 1gb and a 512mb combination, you will be operating in single channel mode. Again, if the speeds differ, the slower speed is what will be utilized by the motherboard.
The best combination is to use two sticks of ram of the same capacity, same speed and same brand/model. The next best combination is to use two sticks of the same capacity, and speed, but different manufacturers. Not all memory chips will work together in harmony. You'll just have to try it if you are using two different brands of memory in the same machine.
Run Memtest86+ to test the memory after installation. http://www.memtest.org/ You should run it for several hours, and see no errors.