Caveat:
I am not modeling or rendering anything in 3D.
I am not doing any video production.
For me, I've never had a need to go beyond 40Mhz with what I use my Amiga(s) for. I have absolutely no desire to "modernize" my machines. I will never try (or want) to run an MP4 video file on my Amigas. To me they are near-perfect time capsules.
7Mhz works perfectly fine for ~90% of all use cases that I will ever touch. 14Mhz (like the 1200) hits another 5% or so. When I get to 25-40Mhz (my 2000 is 40) I frankly have no need to go beyond this. Ever. Does that mean it's wrong to make one go faster? No, of course not. But will most people ever touch any of that power? I'm willing to bet money on a solid "No." In that card on Ebay linked above, will anyone ever scratch the surface of that 128MB of RAM? Nope!
Unless the idea is to try and put a more 'modern' version of WB on your machine, which ultimately starts to crap out on earlier software made for Amiga, it's a race to nowhere. That's been my personal experience.
Now - does that mean that geeking on hardware is a bad idea? Not if you find pure enjoyment out of installing hardware just to see what it can do. But once you're done doing all of that you may find you almost never need it.
I've personally experienced this first hand. When I first got into Amigas I focused SO much attention on making it faster and whatnot. It wasn't until over a year later that I truly realized my personal dream machine hardware upgrades.
In this order:
- Hard drive: 4GB is PLENTY. Hell, 400 MB is fine, too. The larger your drive, the longer your machine will take to boot up. too. Don't go crazy. Most software isn't that big.
- RAM: At least 2MB Fast RAM. I have more (16MB) but seriously never touch it. I think somewhere between 2.5 - 4 MB is way more than most will ever need for a retrocomputing machine that does 99.9% of all you'll ever need to enjoy it. 8-16MB is gluttonous. (I'm a glutton.) Over 64MB (which I have on my 1200, which I don't even use these days) is unnecessary for nearly everyone. There might be 1 guy somewhere, but it's really rare. It's not a requirement. If you do a ton of file transfers from PC to Amiga (again, most don't) you don't need to go crazy here. No games that I'm aware of (that matter) ever tap into more than 1.5MB of RAM.
- Processor: As I mentioned before, most games were aimed at 7Mhz and work just fine. I have personally found some games do play better (in my opinion) with a faster processor. 14Mhz - 40Mhz range is it. And 14Mhz is just about right for 95% of the titles out there. In a few very rare cases, I believe 40Mhz feels awesome. But it still feels good enough at lower rates.
Just my 2 cents. I do enjoy playing with hardware. And I admit it is very addicting. But it's not necessary to do what you probably want to do: play games. They just don't need that much power. And the 4000 is sick by default.