- When powering the board with USB, what voltage and ma come out of the pins? what about the power pins, the 5V and 9V are these usable?
The USB board and the serial board use a 78M05 regulator for the 5V supply, which gives potentially (sorry for the bad joke 
) 500ma of current at 5V. It will run hot at that current, so it's good to use less current if possible. the Arduino uses perhaps 50-100ma, so it would be safe to draw 200ma from the 5V pin, making total consumption about 300ma. See question #3 for why.
- What are the specs when powering from a DC adaptor? eg. 9V 500mA?
yes that will do the trick.
- I have read somewhere that powering devices, sensor / actuators from the board is not the best way to work because microchips like the ATMEGA8 don't like power fluctuations. Is this true?
Microprocessors like the Atmel and the PIC do not like power fluctuations. If you draw a lot of current quickly from your regulated supply, you run the risk of a voltage fulctuation that will disrupt the delicate task of computing going on inside the chip. The answer is to use lots of filter capacitors to satisfy anything that uses current in 'spikes', to limit by design the current drawn from the microcontroller's power supply, and to never never never never use inductive loads like motors and relays that take their current from the regulated 5V supply. For those, you want a separate supply, preferablt diurectly from the main DC source.
I'm probably asking pretty silly questions, but my electrical knowledge is very limited so any help would be appreciated.
Not at all! We're all here to learn and share...