Let's see. My first PC upgrade...
That would have been when I installed a modem into my first PC. Not rocket surgery by any means, but back then it was a tad more complicated than connecting a modem these days.
The computer was a snappy 12.5 MHz 286, with an impressive 1MB of RAM. It had a 5.25" floppy drive, and a spacious 20MB HD. It had monochrome Hercules graphics, and the output was displayed in crisp orange/black on a fairly small monitor. It was running MS-DOS 3.0.
All in all, it was a pretty nice computer, and I sometimes wish I'd kept it in working order (the case was huge and heavy, and for storage reasons I'd dismantled it, keeping only the core components. I also poked and prodded them a lot, doing things like pulling out DIPs and examining them closer. Unfortunately I poked (or maybe prodded) a clock crystal a bit too hard, and it came loose. I've been toying with the idea of soldering it back in place, but I'm not sure if it'd be worth the effort...). (Similarly, I also regret selling my C64...)
Anyway, the upgrade...
The modem was a 300 baud modem handed down to me from an uncle. It didn't connect externally through a serial port, but was instead located on a separate ISA card, so you had to open up the computer in order to install it. The computer case was very tinkering-friendly though, since the lid was connected with a hinge in the back, and locked in place with two push buttons in the front (one on each side). So, to open the case, all I had to do was push two buttons, and lift the lid. That was pretty nice actually, and was probably one of the reasons I quickly began to feel comfortable poking around inside.
The upgrade really wasn't any harder than installing a new graphics card nowadays (hardware-wise at least. Software-wise... Well, that's another situation...), but that was my first solo PC upgrade, and I was pretty happy with the results.
We didn't have access to any newfangled "World Wide Web" at the time, but it did allow me to connect to BBSes, and to participate in Usenet newsgroups. Ah, the good old days when the limiting factor in *chats* was the speed of your connection...
I learned a lot on that computer, both in regards to hardware, and in matters of software. It was a great platform for experimentation, and I tried to take advantage of that. I also learned the secrets of DOS (who can forget the joys of autoexec.bat and config.sys?
Especially back when different games required different optimizations; for instance, some games wanted extended memory, and some wanted expanded memory.), and played around a lot with all the various programs that came with it. I've had quite a few computers since then (most of them I've built on my own), but that first PC was one of the most significant ones (in terms of what I learned from/on them).
I also owe a lot to the uncle that gave me that modem (the modem being only a minor reason). He was the person that introduced me to PC computers; he had a PC, and my brother and I used to play early PC games on it when we were visiting (and I also later got my first views of Windows 2.0). He taught me the basics of DOS so that I could handle his computer myself, and when I got my first PC, I called him a lot in the start, asking numerous questions. It didn't take too long until I became (mostly) self-sufficient (and not too much longer until I generally surpassed him in PC knowledge), but I can't help but wonder how different things could have been if not for all that early help. It gave an environment where I could experiment a lot (if anything went wrong, the worst that could happen was that I'd need to perform a quick reformat and start over), and if anything did stump me, I could just call for help. That allowed me to *learn*, rather than "merely use" PCs. Since I've been using PCs *a lot* since then (and those interests have also been directing quite a few life changing choices), things could have ended up very differently for me if not for that help.
His kids are quite a bit younger than me, so I've tried to repay the favor by providing similar help to them when possible, but since my uncle has been more than able to handle most of that, that need hasn't come up very often. His son is starting to study Comp. Sci. at a university level this fall, so maybe I'll be able to be of some service then...