USB RAM is quite cheap in my opinion (64Mo and 128Mo at least [about 50 Euro each depending on the trade mark]). I don't know how much compactflash will cost.
In addition, there is some MAX (that is an electronic trade mark) componant that do interface USB and RS232. But I've heard that those componants are quite expensive.
It still a dream. I won't try to design such an ATARI extention, don't worry.
