I finished DOOM on my A1200 with a 28Mhz 030, it ran acceptible enough for me to play through until the end, in low-detail. Sometimes I had to reduce the window size, but it's still playable. DOOM also runs on a 14Mhz 020, as does Breathless and a bunch of other 3D games, so at least get the A4000 and try them on it as it is before rushing out and purchasing an accelerator. If you get the 128MB RAM card (and maybe remove the 16MB onboard RAM) it should speed up the system a bit without needing a new CPU since the RAM is much faster.
