What toddos said, with the addition that Kindle for PC is the easiest Kindle format to remove DRM (encryption) from. You should be able to download most (perhaps all) of your existing Kindle ebooks to K4PC. See
Apprentice Alf's Blog for a good summary of DRM issues. For K4PC, unswindle is easiest and skindle is less likely to be broken by an Amazon upgrade to K4PC.