The plug-in CPU upgrade is the greatest salesman's lie of them all.
Countless times each day, the half-trained sales people at your local computer supermarket say: "Yes, our computer is fully upgradable, just plug in a new CPU".
Like all really good lies, the chip upgrade lie is actually true — about two times out of a hundred. Hey, let's be realistic: sometimes the cheque really is in the mail, sometimes I really will love you in the morning. But cheques and true love are both much more common than practical, successful chip upgrades.
There are three key reasons why chip upgrades are usually impractical:
Very few new CPUs are compatible with older motherboards. Sure, you can change the chip, but only to another very similar chip, which won't be much faster anyway. If you happen to have a typical Pentium-133, for example, you can upgrade the CPU to a 6x86-166 or Pentium Classic 166, maybe a Pentium Classic 200 — none of them much faster than your current CPU and all of them long since out of production anyway. You can't use a Pentium-II, or Celeron because they don't fit in Socket 7, and you can't use a 6x86MX, Pentium-MMX, K6, or K6-2 because these need different supply voltages and (at the very least) a BIOS update. Yes you can "just change the chip", but not to anything that's fast enough to be worth the trouble — and in most cases, not to anything that's actually available anymore. (This was written perhaps 6 years ago. Today, nothing has changed. Cross out "Pentium-133" above and write in your current CPU. It's just as true today as it was in 1997.)
Motherboard technology improves just as fast as CPU technology. Often there is little point in plugging a fantastic new CPU into your old, by-now low-technology motherboard, even in those cases where it's technically possible. Take the Pentium-133 example again, and imagine that by some miracle you can get the board to take something more current (maybe a 6x86MX-300 or a K6). Your system remains old-tech because you won't have most of the next generation motherboard features: no AGP, no 100MHz bus speed, possibly no SDRAM support, certainly no ATA-33 hard drive support, rather primitive Plug and Play, a slower, old-tech chipset, and a 64MB effective RAM limit. In short, you're very little further forward. (And just in case you think that the newest gear is different, think again — these exact same sorts of issues will hold you back if you are looking to upgrade your P 4 2.4 or your Athlon 1900 to the latest chip.)
It is hardly ever cost-effective to just change the chip. If you have a standard system, we can fit a brand-new motherboard and any CPU you choose at a surprisingly low cost, because your old board and CPU are a matched pair: they are readily salable and we can give you a good trade-in on them. But just a CPU chip on it's own has little value. To sell it we have to pair it with a motherboard, which in practice usually means buying a brand-new one. But it's still only a Pentium III 1000 (or whatever), so the combination is not worth much more than the motherboard alone.
There is only one good way to upgrade your PC: that's to change both CPU and board at the same time. To be able to change boards, you must have a standard form. That means ATX. When you buy a non-standard computer like an Amstrad, Apple, Compaq, Commodore, Dell, Hewlett-Packard, IBM, Olivetti, or Packard-Bell, you are locking yourself into non-upgradable dead-end technology.
Those supermarket things cost more, they are usually under-powered, with low-grade graphics and the slowest cheapest hard drive available, and they are incredibly expensive to repair. Lots of people buy one of the supermarket things for their first PC. Hardly anyone ever buys another one.