Define R5.
Once something occupies the optical drive bay, the 450D's clean aesthetic is ruined. I mean it looks hideous. With the Define R5 you can hide it behind the door.
Either options will require sacrifice though. Both cases will require the re-positioning of the drive cage to accommodate a 240mm radiator in the front. This will eliminate the possibility of SLI or CFX with long graphics cards, and installing any long power supplies (1000W+) becomes out of the question.
These restrictions won't apply to Carbide 540, thanks to the dual chamber design. But it seems to be out of your price range.