Stay with what you have. Your existing accelerator has a much faster memory bus, can take more ram and has a scsi option I believe. Unless you do a lot of fpu-related stuff eg rendering, image processing, the its quite possible the A3640 will in fact feel slower in day to day operations.
If you want a 64040 upgrade then look for a warp engine, or a phase 5 68040 card or a GVP board. Better still consider a 68060 board.
