I haven't upgraded in years and right now I am saving up and looking around for the best bits to build a new system around.
My current system is an ancient AMD chip at about 2.4Gh with a single core with a cheap as 512MB AGP graphics card.
Although I do have a KVM switch with 4 ports, so I actually have four computers and a network switch to link them all together though I haven't gotten around to setting that up yet.
I do have cold swappable racks in my main computer and three seperate hard drives set up to run as my C: when I want a change.
For those not tech inclined a KVM is a Keyboard, Video, Mouse switch, which is a small box about the size of a router with lots of plugs and cables that allows you to connect... in this case 4 computers to one monitor, one mouse, one keyboard, and in the case of mine also the speakers and microphone, so I can get 4 computer boxes and plug them into this KVM switch and then connect one monitor, one Keyboard, and one mouse and one set of speakers to the KVM switch, so I can turn on all four computers and then either push a button (1-4) on the KVM switch to jump between each box. The computers are not linked together so you can't pass data from one to the other it just means I can have 4 different computers with 4 different setups/operating systems/purposes with just one mouse, one keyboard, and one monitor without having to plug in and unplug all the time.
The Cold swappable rack is like a drive tray that you connect the C: cabling to so that you put in a HDD and set up the computer with operating system and programs you want and then you shut it down and slide out the HDD in its own little tray and put in a new HDD with its own tray and set the computer up all over again but to different specs... so you can have different operating systems, different programs, even different email and internet settings. One could be for work and the other for games and a third could be for when the nieces and nephews come to stay that has very limited internet access and simple games and no dodgy internet history or links.
It means you can use the same computer hard ware... buy a new tray and new Hard Drive and install the latest Linux distribution and actually give it a proper test safe in the knowledge that all your games and important programs are safe on another drive.