For me, it went like this:
1. ZX Spectrum 48K - this was my first home computer. It came with a lot of games (almost all of them pirate) and some programming manual. I was still pretty young at the time so I didn't really use it for anything more than playing games. The games were fun but the loading times were pretty atrocious. It was my first home computer so I have a certain sentiment about it.
2. Pentium PC - the next computer that I got was not an Amiga or Atari ST, but a Pentium PC. I got it rather late and it was somewhat slow at 3D graphics due to it's ageing video card (S3 Virge). But back then, I didn't yet know that I could have added a new 3D card like the 3dfx Voodoo and more memory to the system. This limited the gaming potential of the system but I was still able to play some a little older or less demanding games and also do some computing stuff on it (no internet access yet back in those times though).
3. Pentium 3 PC - this is actually a little inaccurate as it was a "Celeron 2" rather than the P3, but it was basically the same chip. This was the first PC where I got a new graphics card for it (GeForce 4 MX) and it was able to do some gaming pretty well. At the time, I also became more of a PC gamer than a console gamer, despite growing up with the consoles. This was also the first time I got internet access and experienced the magic of it (although the 56K modem was awfully slow and expensive).
4. AMD Athlon PC - because the previous PC started to struggle running "next-gen" PC games like Doom 3 and Half-Life 2, it was due time for a new machine. That machine was a spanking new Athlon XP build with a Radeon 9800 PRO card, basically the best build you could get at the time. It ran games pretty nice, although I did have to get more memory for a stutter-free experience. I also got broadband internet at the time so browsing the internet became a pleasurable experience. I used this PC for quite a while, changing just the graphics cards (first to GeForce 6800 GS, then 7600 GT) but I didn't play the latest games on it as it became dated and could not keep up anymore (at the time, I invested into a "Xbox 360 Elite" for my gaming needs, but I soon discovered that console gaming isn't for me anymore).
5. i5 PC - when browsing the internet and doing other regular things became a big problem, it was time for a new machine. I went with Intel again and got a i5 build and later on added a GeForce 750 Ti card to it. I still use this system as my primary one and I'm pretty pleased with it as it runs anything I throw at it, including compute-hungry tasks (although I don't do things like bit coin mining and similar extremely demanding things). I'll keep using it untill it breaks or won't be able to run things decently anymore, upgrading just the graphics card and adding more memory and storage to it.
6. Intel Core2Duo PC - this is actually a weird one as I got it pretty recently rather than in it's prime time, but it serves me as a server rather than my main build.
I also still repair/update my old PCs and didn't throw them away (except the very first one which was replaced - kind of a bummer because I'd like a "proper" DOS machine too

What about you, what kind of upgrade path did you pick or had to accept?
Write away.
