The collected works of RussianE39

You can drop even modern _PC_ ATI AGP cards into ZX6000 (just make a power feed for them. I'm running HD4850 in my ZX6000 with no problems).
Also you should note that Sun used two versions of 13W3 - one for older machines like your SS20 (which is not compatible with SGI), and newer 13W3 which IS compatible with SGI pinout.