I've just ventured into the world of foliage: for the cards I modeled leaves and branches, rendered out a diffuse, then applied a material with NormalTexMap in the diffuse and rendered out the normal map.
Feanix: Your leaves are far too dense. When you make your diffuse/alpha make sure that you have a lot of negative space between each leaf. Can't really say how it affects the overdraw, but it certainly makes your trees look a lot better. Also, try this: http://www.polycount.com/forum/showpost.php?p=1162581&postcount=13…
Wolfire's examples indeed don't show it very well but the method should work rather well for photosourced leaves. Personally, I didn't have the time to test it myself, but its principles are similar to what I'm doing. When making my diffuse I always make highpoly branches and have the leaves point into readically different…