Sweet dude. Your going to LOVE your 75G (If not, ill take it ;D). my 75G was my first planted tank. I Love it because its big enough to do just about EVERYTHING in a single tank
. Of course, thats still not enough for me, so now ive got two 20L's :O
Haha. As for hills, personally i dont really have hills. What i do is waves. Dont know why, its just what ive always done (Did it in the 75G. Starts with ~1" of substrate on the right, and then on the left theres 5" or so). Works great and you dont have to worry about it running off anywhere.