next lens choice for beginner

I have the e-510 with kit lens 14-42 and 40-150.  I also have a OM 50 f-1.8, which I like very much.  I am working on getting used to these but I was curious what you all thought about my next lens, if I should even bother.

I like to try and shoot landscape and wildlife, but I really enjoy the look of macro photography.

I was thinking of either the zuiko 70-300mm or Tamron 75-300mm, but I was also wondering if you had any ideas about macro lenses.  I have my eye on a OM 20mm f-2 but do not know if it qualifies as "macro" per se.

What do you all think?