If you want to run an A3640 in an A3K, you can - providing it is rev 3.1 or higher. Also, you will get a HUGE speed boost by overclocking it to 33Mhz. The actual performance increase is a lot more than just the additional 8Mhz. Due to motherboard timing issues, 25Mhz on the accelerator bus does not work well on the A3K. The extra speed helps the bottleneck from the lack of ram on the accelerator. Do make sure you put a good cooler on it, a 486 heatsink/fan combo works like a charm, fits well, and is cheap.
A good, cheap (relatively speaking) graphics card for the A3K is the Spectrum 28/24 from GVP. You can find them on Ebay for ~$100 or so. The Spectrum 28/24 has a pass through for native modes. You need a monitor cable and run from the VGA port on the A3K tp the input on the Spectrum, and since the 3000 has a built in SD/FF, all your native modes show nicely (and automatically) on the same, inexpensive VGA monitor, making the integration of RTG seamless. The Spectrum is not the best or fastest card around, but combined with the overclocked A3640, it makes the A3000 quite servicable. I had that exact setup for a while and used it to surf the web, IRC, run photogenics, and even play MP3's. DoomAttack is quite playable on it. I upgraded from that to a CSPPC/CVPPC...I missed the Spectrum, even though the CVPPC was vastly superior...the ease of use of the spectrum was top notch.
The A3000 was a brilliant machine.
If you get any accelerator with a scsi controller (CSPPC in particular), google INT-2 hack because you will need to make a mod for it to work correctly.
check out this page for detailed information how to o/c an A3640.
Overclocking the A3640