Very nice tips, I didn't know that of the BIOS and the HDs.
May I add something? This is according to my personal experience: if you're going to upgrade the processor and/or RAM, check that your motherboard supports the candidates. If you have an old motherboard made for Intel processors, supposing a PIII or PIV, it's very unlikely that it will be able to hold a new Dual Core, the sockets differ. Same with RAM, old PCs may be using DIMM or DDR memories, newer ones use DDR2 or DDR3. In case you have to buy a new motherboard, be prepared to spend some money, it's the backbone of your pc and it's preferable to let go some bucks more and get a good reliable product... I have personally seen good agreement between price and quality in Asus mobos, but that's me.
Then, take into account future upgrades, like extra cards and RAM slots. If you're not going to put more than 4GB or RAM and are not going to use video processing demanding software, a basic mobo will do (2 slots for RAM sticks, one extra slot for a PCI card, built in video, sound and network). About the processor, for everyday tasks and even for some hungry apps like 3D modelers and CAD softs, it's more important to have a good amount of RAM than a mind-blowing processor. I have a Sempron and 2GB of RAM and my machine performs as good or even better sometimes than Athlons with 1GB of RAM. RAM makes for me easier having a lot of windows opened with several apps, and is a must if you are going to virtualize. A good processor is useful mostly if you usually make very demanding number-crunching tasks, like renders or FEA. I have made some and because I do it seldomly I don't miss a better proc, but if you do it often I reccomend considering more than one core.
Last but not the least, check in PCLOS hardware database if your future machine is supported by Linux. Once you install it, spend some time tweaking it for max performance (I looove doing that!)... unless you fancy flashy looks and appearences, like Compiz and alikes.
Well, those were my 2cents, I hope it helps.