I had the advantage of reading through this before deciding on my list.
1. Upgrading to broadband from dial up. This was huge. Changed the name of the game from dialing to specific bulletin boards to surfing the web. Remember when the local numbers for the board were busy? Other house members picking up the phone? The beeping and white noise sounds? How freaking slow it was? Needing to decide – I want to go online now (because it took a few minutes just to connect and start to get something done – as opposed to automatically getting any information online, from the weather to the TV listings, because it is the fastest way.
2. Getting a modem. I rank this as number two, because I found bulletin boards and dial up internet access to be nice, but not a game changer. It wasn’t until everything was fast and always on that I changed my way of life to use the internet for everything.
3. Changing from disk load to HD. In my case – from an Amiga with disks to a 486 with a HD.
4. Monitors. From CTR to LCD. From one to two. From small to large. If I had to pick one particular upgrade – about 10 years ago I got a 19” LCD… it was on sale for like $600, and was usually closer to a thousand. Lasted me, first as a primary monitor and then as a secondary until this year when I upgraded to a 23”.
CPU and GPU performance increases coincided with all of the above, but no particular change was revolutionary to me. Even the switch from no GPU to having a GPU wasn’t huge IMO. If I recall correctly, Quake ran fine on a regular Pentium, no need for a 3d card. And Quake was a fully 3d environment.
My first computer was a Commodore 64, followed by an Amiga and then a 486. So I never really experienced black and white computing, or a computer without sound.