your existing setup is 135W--assuming standard household volts of 120V and using ohm's law:
W=VxI
I=.875A
at 135W your ampergae goes to 1.125A--assuming the fixture can handle that I feel confident the wiring is more than up to the task so I would think it was ok--Again this all depends on the fixture--its bound to be the weakest link so you should see if there are any restrictions on it.
Actually, your Ohm's law example only works for DC circuits. Ohm's law is different (and more difficult) for AC circuitry.
Either on the box or on the fixture itself, there should be a label that tells you the maximum wattage of the fixture. Once you have this, just make sure the wattage of your bulbs added up is equal to or less than the maximum wattage of the fixture. No fancy mathematics neccessary.
The problem with overloading fixtures is that the wiring can get hot and even catch on fire. We don't want that now do we?
