This is (currently) not possible using the web interface.
I'd recommend going through and deleting things you don't need in each folder, perhaps sorting them into folders such as "Keep", "Not Sure" then use selective sync with your PC to only sync the files you need. Though this is not very practical on the web interface as you cannot move around files by drag and drop. Downloading everything to your desktop is your best chance of sorting the files in your Dropbox.
Apart from traffic caps imposed by your ISP or limited local hard drive space I can't think of any reason why you would not download the whole thing and sort it there.
Of course you can always get one of your family or friends to accept an invite to join Dropbox from you to gain a 500mb bonus.