Watts are easy, just add them together.
Your previous set-up using 4X20W bulbs would be 80W, drawing 6.7 Amps.
The new set-up with 6X10W bulbs is 60W, drawing 5 Amps.
12W bulbs is 72W (6A), 20W bulbs is 120W (10A).
Look on the transformer for the maximum rated load.
Of course, you could invest in LED bulbs click here