Now questions :
- 1) Can I connect to R-Pi via SSH (not sure about if SSH) and upload there videos through wifi on ext. HDD ?
- 2) Do I understand it right, that I have to convert every video to H.264 ?
- 3) Would R-Pi be capable of running XMBC and some more (etc. Torrent client, File Server ...) ?
Now answers :
1. Yes by using an scp client such as Winscp for windows. I've been using this to upload some addons to my OpenElec version. Please note however that using WiFi and SSH with the encryption overhead might not be very fast. ftp might be faster if you can get an ftp server loaded on it with the assumption that its on your private network so security won't be an issue.
2. Depends on what the current format of your video is. The Pi supports Xvid/Divx and x264 natively.
3. Maybe although be aware that xbmc is fairly heavy on the CPU for the rendering of the menus which is reduced somewhat by using dirty region settings. Also the memory is typically split 50:50 with the GPU so that might also be limiting. You may want to look at the raspbmc distro that Sam Nazarko is building (not released yet) as I believe he is intending it to be extendible.
Hope that helps.