The applications are installed in the internal flash, not on the cards. The internal flash is much bigger than it looks like, because it uses a compressed file system. I have tons of apps installed and there's still lots of space. No way you're going to run out of application space with anything like normal use. Just make sure you put big data files (media and music and the like, and maps etc.) on the card(s) instead of internal flash.

The reason you can't install applications on the SD cards (out of the box) is that they are FAT formatted, and the braindead Windows FAT filesystem doesn't have support for the 'executable' bit (only read- and write bits and such) (and Windows use the filename extension instead to identify executables - talk about silly).

But then again, with normal use there's no problem with the size of the internal flash. If you need to install applications on SD, look up 'clone to MMC' here on ITT, through the search function.

As for repositories, go to http://gronmayer.com/it/