as you already have some fast ram, i'd say it is down to what you want to get out of the system.
i did most of my most productive stuff with an A1200, a 4MB fast ram card and and 20MB hard disk.
soon as i started with accelerators, mega amounts of ram (in amiga systems), graphics cards, i kinda lost sight of what i was doing, and for why. i ended up jumping on the bigger better faster upgrade cycle with no real purpose except that i really wanted these things when i was a kid and couldn't afford them. not that i was really running any software that really needed the extra horsepower.
then i started getting into tweeking around with imageFX and photogenics. then on to extracting frames from video, deinterlacing, applying a filter or two, then re-insert the frame back into the video. all under AREXX control. overnight batch processing.
anyway, accelerators it seems these days are an easy way to boost the ram on machines, and extra ram is a bonus for WHDLoad. its not like anyone seems to use miggys for CPU intensive stuff any more.
hell, my 030 based A500 can outgun my A4000, in yet all i use it for is WHDLoad Lotus2, flight of the amazon queen, and a bit of monkey 2. populous1/2 seems to be cpu speed locked, so anything over a 7Mhz '000 is a challenge!
but if you are interested, a basic break down follows
cpu speed compaired to basic A1200
1x A1200 (14Mhz 020)
2x A1200 (14Mhz 020, fastram)
4x A1200 (25Mhz 030, fastram)
10x A1200 (50Mhz 030, fastram)
20x A1200 (25Mhz 040, fastram)
40x A1200 (40Mhz 040, fastram)
80x A1200 (50Mhz 060, fastram)
100x A1200 (66Mhz 060, fastram)
plus running something like fblit and ftext will make window/icon redraws and gui operations so much smoother/faster/responsive.