When calculating watts per gallon...

Watts per gallon is a rough way of estimating how much light you need based on the size of the tank, not how much is actually in it.

If that's the case, I have less than 2w per gallon. I have 1.8333..

I have 4x55w in a 120. Looks like I'll be making a lighting upgrade.

many have observed that high WPG is less important with a larger tank,
when going after high tech plants. also the tank height make come into play.
if your tank is 18" high then your current lighting may be sufficient.
if it's over 24" high, then perhaps more light WPG would be helpful.

It is 24" high. I'm thinking of just going with 65w bulbs...not a huge change.

I wouldn't bother. 55 and 65 Watt PCs are the same bulbs, just driven by different ballasts. It wouldn't make much of a difference.

For what it's worth, I have a 120 lit by four 54 Watt T-5s. It's definitely not high light, but I have no problems getting stuff to grow. What plants do you have in the tank? And how heavily is it planted? That can make a big difference because a densely planted tank will mean a lot more shade.

reflectors combat the issue of the height of your tank. If you get parabolic reflectors 24" will not be an issue. Ask anyone on reefcentral about using t5ho's with parabolics on tall tanks, some will tell you they penatrate just as deep as metal halides. My tank is 20" tall and when I switched from crappy jbj 72w to the sunlight supply 78w the light was SO MUCH brighter and penetrated much deeper. the difference between 20" and 24" is not that much. Now if you had a 32" high reef tank you were converting into planted you would need a more expert opinion then mine.

By the way what substrate is that?

SMS substrate. I do have reflectors.