After the xbox is soft modded can I load roms and emulators from a sd card or usb thumb drive? Can I load movies from the sd card? is the ps1 emulator as good as the psp emulator for playing ps1 games?
Sorry there is no real soft mod for the 360. There is the firmware mod (or DVD flash) which can only play backups from disc. If you want roms/emulators from a HDD/USB drive you will need to hardmod it (RGH or Jtag).
No you can't.
USB is locked at 1.1 and is just not doable unless you want slow read/write and can be up to the task of soldering a USB port to the controller port.
SD loading simply cannot be achieved since it lacks a SD port/pinouts for an SD port.
Just upgrade the internal HDD drive or burn stuff to a DVD (or upgrade the internal HDD after you dumped your HDD key)
Edit, The emulators are pretty good, don't expect every game to work tho.
Movies can also be played with XBMC 4 Xbox.