The heart of any computer is its processor, and whilst it's often not the main bottleneck preventing the machine from giving its best, replacing a PC's CPU with a faster one will make the whole system quicker. This, the latest in Intel's range of "Overdrive" CPU upgrades, is an 83MHz Pentium for 33MHz 486 motherboards. It won't turn your old PC into a full-speed Pentium: there are a number of technical difficulties in replacing a 486 with a Pentium, the most awkward of which being that the Pentium has a 64-bit bus whereas the 486's is only 32 bits wide. This means that a special version of the Pentium, the P24T, had to be designed to fit onto a 486 motherboard, and even then it requires a special socket. It runs internally at two-and-a-half times the clock speed, which means that it makes heavy demands of the machine's second-level cache. Whilst a 64K cache will do, 128K is better and more is desirable; it will perform very badly in a machine with no secondary cache at all. Furthermore, for best results, it should be a write-back cache (which speeds memory writes as well as reads) as opposed to more common but slower write-through variety (which only caches reads.) This means that the full benefit of the upgrade will only apply to a modern, well-specified 486s, although lesser machines will still be accelerated. Finally, the Pentium has greater power and heat-dissipation requirements than a 486, which at the end of the day restricts the number of PCs in which this processor will really shine. Intel have a list of some 3,500 systems it has tested and approved, but these are merely those in which it will work at all, not those for which it's best.
I tried the processor in four different machines, and it worked in all, but the speed gains varied widely. An IBM PC Server with a 486DX2/66 and 256K of write-back cache increased markedly, whilst an AST Bravo 466LC desktop, with only 64Kb of write-through cache, was less impressive. Older 33MHz machines did well too, but the older motherboard designs mean they can't quite catch up. Intel claim that a P24T is a better upgrade for an 8Mb DX2/66 than another 8Mb of RAM, and while this is true in that the speed increase is more than the RAM will give, 16Mb is a wiser upgrade - many programs run badly, if at all, in 8Mb of RAM, especially Windows 95, which really needs 16Mb. Overall, a 486/33 machine should run about 60% faster, and a 66MHz PC will gain about 30%, but it's not as fast as a "full" Pentium-83 would be - it's closer to a Pentium-60. If you already have a DX4/100 then it's definitely not worth it. The case for other machines is less clear-cut, but for Windows users, it's definitely worth upgrading to 16Mb first. If you have a power-user's 486 machine from a couple of years ago, with a DX33 or DX2/66 plus a P24T socket, 16Mb of RAM, a VESA motherboard and graphics card, and enough hard-drive space, then it's a good deal; whilst the best alternative would be swapping the motherboard for a PCI-based Pentium one, this will mean you'll need a new graphics card and quite possibly new SIMMs too, and so will cost substantially more.