I like building up computers. Something about it just makes it more 'yours', and something you can have a bit of pride in.
I currently have a slightly old gaming rig and a backup file server. The gaming rig is built for strategy games rather than RPGs, so the graphics card is a little dated (Radeon 4750 IIRC). Unfortunately when I built the computer it was when the first Intel i7 CPUs were released, and slated to be a solid pin-out to guarantee an upgrade path...
That lasted until the first i5 series was released a few months later. Intel changed the pinout for the i5 and the subsequent i7s, destroying any CPU upgrade path for the system I had built.
Even worse for the upgrade path the original i7 chipset supported triple-channel RAM, and all of the newer models reverted back to double-channel; so when I upgrade the core of my computer I will have to change the motherboard, processor and RAM all at once instead of being to apply a rolling upgrade. Luckily the system I have built is solid, and should last a couple more years now the main HDD has been replaced with a 500MB/s r/w SSD, and the next bottleneck should be the graphics card. After that it is likely to be a brand new build.
My file server (separate PC) was built to be low-power. I specifically hunted round for suitable components and the final build, under normal operating conditions, costs around £30 per year in electricity. Not bad for a headless i5 base.
Every now and then I resurrect my old Psion 5MX (released in 1999). I think it is absolutely amazing that it can run for over twenty hours on just two AA batteries - with a decent touch screen and backlight. Admittedly it is only grayscale and hasn't got WiFi, but it is still a fantastic piece of kit to play with.