Approx. 60 - 70lbs. will give you about 1.5" in front and 3.5 - 4.0" in the back.
A great substrate for a planted tank is 100% Flourite. An almost as great substrate IMO, and more cost effective, would be 45lbs. of Flourite and 20 - 25lbs. of the cheapie beige colored gravel (very similar in size to Flourite) that practically every LFS carries.
Len