I admit, I stopped reading when I hit streaming
Most first time forum users mistake the Live with the Live Streaming and it seemed the same case...
Regarding answers to your questions, here goes:
What is the path name for the usb stick? I mean how shud I structure the usb for the WDTV Live to be able to find the relevant moviesheet data of a certain movie from a shared folder in my serverpc? Where do I tell Thumbgen to save them?
You can find out the path to the usb drive (from the wdtv's point of view) either in the webend mount table (it is something like /tmp/media/USB/...) or by running the mount command via shell.
You can have the moviesheets copied to the USB drive (by enabling Samba sharing in the webend) from the PC (and point thumbgen to that share), or you can leave them in the video's folder and use the linksheets script to automatically detect them. See:http://wiki.wdlxtv.com/Net.mountshttp://wiki.wdlxtv.com/How_to_use_Movie_sheets_v2
I also want to use a new OSD theme and I gather this shud also be saved in the usb, right?
Yes, put it on the root of the drive and keep the drive connected.
Do I have to delete from tthe root directory the 3 files used to flash the player after its done?
No, or it depends. The player will display the new firmware found message only if the version number is higher than the current version number. If it is the same, it will simply ignore the firmware.
Finally, I guess an 8gb usb stick shud be more than enough, right?