I am using FreeRTOS together with a comercial graphics library, from Ramtex (http://www.ramtex.dk/). Works fine, was not expensive, and has a beautiful simulator. I develop UI software in GCC (and Eclipse) under windows, run it with the simulator, and it can then be easily moved to target (AT91SAM7S256).
I have no exact figures of footprint, but it is much less than the limits you say.