Dunno about turning an A1200 into a PC-beater these days. The fastest accelerator available still trails fast PCs in sheer horsepower by some way.
Still, the A1200 is a great machine, and I am of the upgrading school of thought.
My rule is: upgrade the machine to fit your own requirements. Fed up of floppies? Get a HD. Want to boost overall performance? Buy an accelerator and some memory. Want to upset rattus? Tower case.
The more you put in, the more you get out. Even a little extra memory will provide a big boost in performance.