If you are going to add RTG to the card then might I suggest something cheap and well documented with tons of example source code for the driver writers: The same chip that the Raspberry Pi uses.
Umm... The Raspberry Pi uses an SoC. That's System on a Chip in case you haven't heard the term before. It means the CPU and GPU are both on the same chip. Surely you don't want the ARM v6 processor introduced into the design, do you? Also, only the user-level registers are documented publicly for the GPU. The video BIOS for it is still a closed-source binary blob.
