just like with other lights that you use in your tank, the more watts the better, well within reason. me and my friend have the same setup, except for the light on our fuge... ive got a basic strip flourescent with a grow light bulb in it, and my fuge is doing ok, and everything is growing. well in his he is running a 65 watt PC fixture over his, and he has so much more growth than i do!!! the growth is actually about 3 times the growth that ive got in mine. the moral of this story is that just about any light with the right spectrum, around 6500k will work fine, but the more watts you have the more growth you will recieve...
oh by the way lowes sells small round metal fixtures that will take a screw in bulb, kinda like a shop light, just smaller. they are very inexpensive, im going to get one for my fuge soon. and they have the correct spectrum bulbs to fit it... i think that the fixture was about 60 watts for around $10-15!!!