Ebook reader question

I am thinking of buying an ebook reader but my problem is that I have half my books on Kindle and half on Epub. Also, a bunch on PDF, Mobi(which plays on Kindle). Is there any ereader that can handle all the different formats or do I have to get a Kindle and a Nook?

Also, is it possible to take a book in one format and somehow convert it to another?

Duplicate OP. Original here.