Has it gotten dimmer since you got it? Or you just think it's dim. If its gotten dimmer since you got it, it could actually be a problem with the the ballasts inside your hood, as opposed to just the light.
Anyways, it obviously depends on the fish you have in there, but as long as you aren't trying to grow plants then 20 watts on your 30 is fine. and as long as you don't have any fish that LOVE light or anything. lol.
if you personally just want it brighter you can look at the different types of bulbs. yours may be only a 50% day light bulb (should say right on it), while they make 20 watts that are 100% daylight and would appear brighter.
hope that helps.
-brent