The only really solid things about the project currently are:
ez8 micro: get a 64xx series one, please. The 082A series is smaller, but has an internal oscillator. However, the difference in flash causes several problems for programming for cross-compatibility.
speaker: Get one. It will probably be hooked to the propeller, but that could change.
SD card: See speaker. Same reasoning.
Keyboard: Get a PS/2 female port unless you want to chop off a plug.
Other than that, it's hard to guarantee anything. Most of the other circuitry won't affect compatibility, as long as it works. Video is a difficult question, since LCDs cost a lot of money (read: >$15 for anything good) and the propeller supports VGA and Composite out of the box. Right now, we're thinking VGA, with various text and graphics modes the propeller will handle.
For example, text mode with customizable fonts would make for an easy monochrome tile system, and there will probably also be a color tile system. There will definitely not be a direct 24-bit color bitmap mode, since that would take 230 KB just for a 320x240 resolution, and the propeller chip only has 32 K total. Text modes can be much higher resolution, though, as can certain drawing-based methods, if someone codes them
The method for interfacing the ez8 to the propeller is still being discussed. The ideal, in my opinion, would be a shared RAM area. By using an SPI RAM chip (32k or so) or perhaps by making the propeller's 32k RAM space available over SPI, the graphics could be implemented in a framebuffer, and other devices could be handled simply by writing the correct byte.
