I agree with the whole "WPG" being a bit outdated HOWEVER its always a good measurement to say I have X amount of watts per Y gallons.WPG is an outdated term and doesnt work due to the different types of lighting available. What are the demensions of the tank as in LxWxH and keep in mind what you want your tank to look like down the road because it can be very expensive to buy the wrong lights and then buy the correct lights later.
HUGE vouch for GLO t5 i've got two of em and they are for sure one of my favorites, just don't have a tank for them yet.I am just going to throw my 2 cents in on top of this already great advice. Try to think far in to the future. For example if you ever plan of having "high-end" not meaning expensive but LPS or SPS corals, you will probably want to go ahead and get the ligths that are made for that now. I know personally I would love to have MH because of the lighting effect they give on the bottom of the tank, thats the only reason I want them right now. I personally run the GLO HO T5 fixture and love it to death. In my research they cant be beat when compaired, as far as output goes, to other similar fixtures on the market.