If you want to do stuff other than mess about with games you'll need an accelerator. I used my 060 Amiga 4000 for web browsing and some 3d rendering and for both of these things you need an accelerator (and of course, more RAM). I would say, if you want to do anything other than play small older games you need to whack it at least more memory and a hard drive but your Amiga is going to feel a LOT more modern and responsive with a decent processor in there.
By the way, surely 20% is a bit of an underestimate even if you say "decreases rendering time".
