Hi machinehead
Take a look
HEREAs you can see, the Spectrum 28/24 has a limited 85Mhz bandwidht and ony 1 or 2 MB of VRAM. (you can use a 16 bit mode for the WB)
So you can use only 800x600@24bit with a VSync of 49Hz (too much flicker).
i haven't understood which RTG system you have installed, but i suggest you to try P96 that supports your board as well.
Ciao and Happy X-mas to you too :-)