Best thing you could do would be to get yourself a Mediator and a Voodoo card. Although it's gonna be much more expensive all in than just getting a Cybervision 64, you'll have plenty of future expansion capabilities i.e. Soundblaster cards, reasonably priced ethernet, USB and maybe even Elbox's mythical SharkPPC!
The performance of the Voodoo/Mediator combo is about the best you'll get on classic Amiga.
