A few answers.
1. You can use LaunchELF to copy your gamesaves to USB drive, IDE HDD and to the PC over network. That way you can have "unlimited" space on your MC, there's just some copy work needed. Here's how it's done:
http://www.psx-scene.com/forums/show...852#post351852
2. Yes you can play online and due to the great auto-DNAS-patch you don't have to patch backup games and don't have to disable the CC for original games. But remember that you can only play games of your own region. (unless you patch the PS2 BIOS)
3. LaunchELF (or ULE) is the most powerful homebrew app on earth. It comes with filebrowser, texteditor, JPEG-viewer, HDD-Manager, gamesave backup functionallity, it has network, USB and HDD support, and of course it can launch ELF files. (ELF for PS2 is the same as EXE for Win PC)
Here's the official support thread:
http://www.psx-scene.com/forums/showthread.php?t=37242
And here's the Wiki:
http://ps2ulaunchelf.pbwiki.com/ (it's still in the works and not up to date)
4. Yes, you can use most standart IDE HDDs in the PS2, but not all models fit onto the network adapter (e.g. Western Digital).
afaik, Samsung and Maxtor drives fit well.
You can use a USB HDD or thumbdrive too, it has to be formatted in FAT32 and not all drives are compatible with the PS2's USB drivers.
The other homebrew app you should definitely try is SMS (Simple Media System), a great media player for avi and mp3 files (mpeg coming soon). Check the homebrew forum for some more apps and games.
Official support forum:
http://www.psx-scene.com/forums/forumdisplay.php?f=111
Homepage:
http://www.sms.psx-scene.com/eng/index.htm
If you want to learn more about your CC and the BM check the CC Wiki:
http://ccwiki.psx-scene.com/index.php/Main_Page
The megapack contains most available homebrew apps, but not the latest versions of SMS and ULE.
best regards Jones23