This is actually quite an interesting thread, because everyone uses their system differently.
For me I wanted to try more then just WHDLoad Games, I mean I enjoyed my original 8mb Ram card and playing Games but with the excellent Workbench packs on offer, especially Classic Workbench I saw that there was a lot more I could do with my A1200 then just Gaming.
First upgrade was a 1230 and I'd highly recommend this to anyone wanting to take the next step, it opened a lot of doors for me. The speed boost with 3D Games was noticeable, Workbench really flies and you have access now to some interesting Emulators such as ShapeShifter (yes you can run it on an 020 but it's not much fun)
Again you can use some 8 bit Emulators with just an 020/8mb but they greatly benefit from an 030.
Next step are the 040/060 cards but moreso RTG Graphics as this brings a big boost to Workbench, I chose RTG as my next upgrade path even before an 060 because I wanted a high colour larger working area, with my Mediator/Voodoo setup (required Towering my A1200) Workbench was a real dream to use.
I've been down the 060/PPC path too, you can build a fantastic system but you start to realise that in fact you are just trying to do stuff with your Amiga that a cheap PC or WinUAE can do better.
If I look at all my Amiga setups I've owned then I would pick out two fav's that would be an A1200 with Blizzard 1230, all you need for Gaming and a bit more! And then my A4000 with 040/RTG (WarpEngine/CV643D/Toccatta) This for me is almost the perfect big box setup, it would be nice to have 060 in there again but the 040 @40mhz is fast enough for most things I emulate and I like that this Amiga is classic Zorro only.
There's lots you can do with a system like this, working with Music & Graphics, Emulation (ShapeShifter flies on RTG), Demos, CD Games that use the sound card and RTG and of course it can still do WHDLoad stuff!
Anyway, that's why I accelerate!