The BeBook or Cybook can read your DRMed MOBIs, but you will have to register the new device's PID with your ebook retailer and redownload the files. If you strip the DRM with mobidedrm (not thought by some to be legal in the US and a few other places) then all three devices, including the Kindle, can read the resulting DRM-free MOBIs and you don't need to redownload them.
Only the new large screen Kindle will likely handle PDFs well. On the Kindle 2 the only option is to convert the file to MOBI (AZW). Amazon will do this for you, or you can try Windows Desktop MobiPocket Reader which will import the PDF and export a MOBI (in its default ebook directory, typically My Documents\My eBooks). Calibre has a similar PDF import option. Converting from PDF to MOBI is hit and miss, sometimes it works well other times not. However, you can try it before buying the EInk device. Also, the MOBI will work on all of them.
For "native" PDFs on the Cybook and BeBook, see
Links to screen shots of PDF documents on e-ink device.
Amazon has about the best selection and good prices for ebooks, but you need a Kindle (or an iPhone/Touch) to buy their ebooks. For generic MOBI, I like FictionWise and Books on Board and WHSmiths in the UK (although many UK titles are ePub-only).