Too much light depends on how much work you want to put into your
tank. I have 4wpg on one tank and while it's nice to have it, it's a
lot of pruning and not a lot of margin for errors when it comes to
fertilizers. I would however think that 4 T12s on a 60 gal tank should
be able to grow most plants so light may not necessarily be your main
problem at the moment. I would start with adding CO2 and working on
nutrient levels, once you have mastered this it will be easy for you
to make the call regarding stronger lighting, it will mostly change
rate of growth, nothing more. There is little that can't be grown at
your current light level with good CO2 and nutrients. You can also
consider overdriving some of your current bulbs, will be much cheaper.
If you do still want to increase the lights, perhaps try to stay in
the 3-4wpg range, you can definetly grow anything at this level.

Hope that helps
Giancarlo Podio