The 5mm diodes won't cut it for the majority of plants. You're going to need high-power LEDs. I used 3W Cree XR-Es for my high-light tank. The build is detailed in the link Smitty provided. You could use 1W models for lower light levels. Take a look at the Marineland LED
fixtures. The Single Bright model doesn't do much for plants, but the Double Bright and Reef Bright models will put your tank in the low to medium and medium to high light ranges respectively, depending on which tank you put them on.