Upside is that workbench and the overall feel of the computer could increase 2x-4x with a good 040 accelerator (more with an 060 accelerator) especially one with fast RAM.
Downside is that the 030 is more compatible for WHDload games than 040 but only just. The large speed increase is more than worth it.
If you are still using floppy disks and tools like relokick with an A4000 then you are MAD!
Invest in a copy of
http://www.whdload.de and then grab pre-installed hard-drive versions of your games off the interweb. (search for KGWHD)
You should also look into the Workbench patches such as SystemPatch or WCP etc. plus SFS file-system as your A4000 is undoubtedly running slower than it need!
