I like hairgrass for a grassy look, but mind u that this stuff gets tall.
haven't heard anything bad about moss balls other than they move around all the time. very trendy "plant" at the moment, so u might see some high prices with them
Try java fern and/or java moss. Look at my 25g tank link for some java fern pictures. They both thrive under low lighting condition. I only have 20w on a 25g tank and they do very well.