I don't know how/if it's enforced, but apps are limited to 2 meg in size. For your cache problem, I thought about the idea of starting a slideshow with one huge jpeg that would take up most of the RAM. For what it's worth, even thought it's not mentioned in the docs, the photo viewer supports bitmaps. It's been a while since I tried viewing bitmaps (.bmp files) so I'm not sure, but they may have to be renamed to have a .jpg extension. It seems like a kludge, but it might be worth experimenting with. There's also the ability to create files on the Roku in a tmp directory/partition. There's no documentation as to where the file actually gets stored so I don't know if it gets stored in RAM or if there's some other memory like flash, but I would think it gets stored in the RAM. There's also no documentation as to how much storage space there is in tmp, but I suppose you could try to write out a large file and see what happens.
As to the question of how much RAM the Roku DVP has, I believe there is just one 64 megabyte chip on the board, but don't hold me to that.
-JT
