Nicole is playing very good tennis these days and I think she certainly is capable of going top 20 (but to do that she should've been in top 40 by now, as the clay season approaches), but it's a safer bet that she can make, as you said, 20-30 by years end. She has improved her game dastically this year and changed it so that she can keep up with the power players and adapt to different surfaces etc. I've never seen her play as good as she did at AO, she was in some sort of zone! I think she was so focused there that she sort of burned herself out for the next few events, but is now finding her range again. She really has not much to defend til Birmingham (SF points) and after that nothing of note really. She missed a lot of tournaments toward the end of last year with a foot injury so she will make up some points there.
She should be higher than 53, but I dont pay attention to that cos I know she's bound to lift that up sooner or later.