My take: Bookshelves, definitely.
Affordable and decent, don't spend too much. Bad acoustics will make any great speaker sound terrible, and you will be wondering why you spent so much.
I might even skip the center altogether, and opt for phantom. Personally, Id really want to see the "final" room and setup before choosing my center. Whether upright tower, upright bookshelf, huge horizontal center, etc. Do you even know if it will be on a TV rack, underneath a huge PJ screen, or wall mounted? Of course not.
Basically, I agree with fredk's post.