Some tips on upgrading older machines.

[DrDOS]

Here are some obvious and not so obvious things you can do to upgrade older machines.

Upgrade the processor. This is pretty obvious, but it can make a major difference if you go from a Celeron with a 66mhz front side bus to a P-III with a 133mhz bus and you have memory that can take full advantage of it. Also, you will have double the cache, sometimes even more, which is very fast memory.

Max out the memory. No version of pclos will run well with less than 384mb, but all of them will do well with at least 1gb. And use memory that at least matches your front side bus speed.

Use a hard drive with lots of cache memory. Older 8 - 10gb hard drives had as little as 256kb of cache, Newer ones, starting with the Western Digital SE series, commonly available used, have 8mb, and it can make a huge difference in speed, even in web browsing and watching videos.

Check your BIOS setup to see that DMA ( direct memory access ) can be enabled,  This allows large blocks of data to be transferred directly to and from memory by the hard drive and other hardware. You may find that a BIOS upgrade will allow you to do this. Also make sure your hard drive supports DMA.

[CAVT]

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.