Right, to try to simplify this for you
1) You DON'T need high lighting to grow plants. There are many plants that will happily grow with as little as 1.5 watts per gallon, whilst still been attractive.
2) Almost everything will grow under 'medium' lighting (about 2.5 watts per gallon) with the exception fo plants like glossostigma ellatinoides.
3) High lighting (over about 3 watts per gallon) mean high maintenance. When using such high levels of lighting things like good circulation (current opinions reckon that about 10x the volume of the tank per hour is ideal, this doesn't have to all be from one source though, but can be achieved with carefully postioned flow pumps and the like), regular nutrient dosing (including the macro nutrients nitrate, phosphate and potassium as well) and fairly high and stable levels of CO2 (about 30ppm) become essential to avoid unhappy plants and your tank becoming an algae farm. However this style of aquarium (often referred to as high tech) does allow the growth of some of the more difficult and demanding plants such as alternanthera reineckii and also the more difficult red plants.
Next, how to achieve these levels. From your description you currently have only a single 38 watt T8 tube? If so then you currently have under 1wpg of lighting. Not the end of the world, as this is still adequate for growing plants such as java fern, java moss, cryptocoryne wendtii (and many other cryptocoryne species), nymphae stellata, hygrophila polysperma, hygrophila difformis, quite a few different echnodorus species and marimo (moss) balls. However you might want to avoid any of the more demanding plants, especially most of the red species and also so called 'carpet plants' as your current lighting is unlikely to have the intensity to penetrate down to the substrate. You could improve matters slightly with a 2nd 38 watt T8 tube, and also make sure that you have reflectors fitted to these. This would then give you a bit more leeway with low lighting, and allow you to grow plants like dwarf hairgrass assuming that your tank is 12 inches deep. If it is 20 inches deep then I would recomend chaning to 2 T5HO tubes of 39 watts each. The extra intensity from T5HO lamps is better able to penetrate to greater depths.
If you wanted to go medium light then you would need 3x 39 watt T5HO lamps, if you have a normal hood you can achieve this using external controlers, if however you have a Juwel style flap hood then you then have to look into adding them by resting bulbs on your brace bar, getting a new ibar type fitting, or the best option buying a 3 tube luminaire from somewhere like ebay. If you are going for medium lighting though fertiliser dosing becomes more important, as does the need to provide a source of carbon for your plants via either liquid carbon supplements like EasyLife EasyCarbo or Flourish Excel, or via CO2 injection. You can however get away with using DIY yeast CO2 kit for this style of tank.
For high lighting as mentioned you need some serious circulation, via either oversized filtration or additional flow pumps, powerheads or the like. Without these you get 'dead' spots where your CO2 that you need to add, or nutrients that you are doing, either don't reach or build up in higher than wanted concentrations, and along comes blue green algae. You need to be dosing nutrients and CO2 at higher levels as your plants will be using these faster because of the extra light, don't add them and again algae comes avisiting as it has all that lovely light but doesn't really need the CO2 or nutrients to grow, whilst your plants do. You also need to stay on top of water changes, with a minimum of 1 25% water change a week, this is mainly to keep ammonia levels down as algae loves ammonia. Your plants lover water changes as well, they add fresh supplies of trace nutrients in addition to your dosing. They also help to remove discoloration of your water that builds up over time and can reduce the quantity and quality of the light reaching your plants. By far the easiest way to achieve the wanted light levels for this kind of set up is with an overtank luminaire, these do not have to be metal halide fittings though, a 4 tube 39watt T5HO fitting would do the job just as well in your tank, but you are looking at spending about £119 and up for one of these. Again if you have a normal wooden hood/canopy with enough space you could just buy 2 dual 39 watt T5HO external controlers and fit the tubes into your hood, remembering to fit reflectors to each tube. You also have to keep on top of pruning with this kind of set up, most of my faster growing plants grow an average of about a foot a week if I let them. So don't go for a high light setup unless you have the time and the inclination to put in the work.
I hope that this has been of some help to you, and remember, you don't need high light high tech set ups to grow beautiful plants, they are just the fashion at the moment.
Ade
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