there for the most part seems to be less information on leds for planted tanks compared to reef tanks, which is where my knowledge comes in. i will tell you it depends on what kinda par you are looking to get ultimateley. i would think 60 degree optics will do here, which means at a depth of 20 inches each led would cover a 1 ft radius at the bottom of the tank, you can double lap each one and have two rows of 6 and two leds will be lighting every part of the tank, you could tighten it to 40 degree optics and each leds would only have a 7 inch radius which would put you at about 24 leds, 3 rows of 8, and thats just a starting point. honestly that doesnt seem like much light and you might crank it up from there.
http://www.nano-reef.com/forums/index.php?showtopic=206246
this is a similar build and he used 48 leds for a reef, 24 might do for you, you can add later, thats the beauty of leds. he used 60 degree optics but, he has to blend in royal blues with his whites, you only need whites, or a few blues to whiten the color. cool whites are about 5-7 thousand k.
driving that many, you might think about meanwells since they have a 48v version which could power like 15 each, you could get two and power 30 leds, which is what you could end up doing. there is a lot of variation as each build is individual.
on the other hand if you use buckpuck drivers you could add 6 leds at a time
so get 4 pucks to start and hook up 24 leds and see if you like the growth, if thats not enough throw in 6 more, then another 6 if thats still not enough.
you could cheapen it a lot if you learned how to build your own driver. current limiting is whats important here. you could try to learn since you have time on your hands.
