The general assumption seems to be 8GB again. That's plenty of space for buffering 1080p though not enough space to hold whole 1080p files such as those converted from BD without really pinching the compression. If the streaming functionality is well done, bigger files should simply que up at the computer and be pulled over in the background so that the bits are there when ready for display. Hopefully, that's the case.
I'm sure iFixit or similar will reveal all within about 3 days. Personally, I wish Apple had built in an option for hooking up whatever local storage we desire to a (normalized) USB port and/or built in a SDHC or similar port where we could beef up the local storage space if desired.