The motherboard you buy will determine:
An ATX board will probably be better designed, in that the CPU and memory sockets will be located so as not to prevent the use of full-length cards.
Neat things to look for just a few more bucks, you can sometimes get built-in video, sound, or SCSI, a second CPU socket, larger cache, or extra SIMM sockets.
Although these things can save you money, just keep in mind that you're putting your eggs in one basket: when you upgrade motherboard, you lose your embedded components, and if you want high quality components, you can probably skip the embedded ones.
In the chips:
Know the different chipsets and ask about which one it has.
It's stapled to the motherboard, but chances are that this piece of hardware, which holds critical software, is one of the weak points of your computer ROM's may be either EEPROM, EPROM or PROM.
Get a flash BIOS. This way, since you already have Internet access, you can download newer versions when required to fix bugs. This type of chip is an EEPROM. BIOS is the term for the software stored on a ROM chip. This software runs during system boot.
Popular BIOS add-ons are SCSI BIOS, hard disk auto-detection, hard disk formatter, APM, and Plug 'n Play.
AMI, Award, Compaq, Microid Research, and Phoenix make BIOS's.
For Pentium motherboards, you can choose Intel, Cyrix (or IBM) 6x86, or AMD K5 chips. A typical Pentium CPU sits in a Socket 7. It has a clock multiplier (externally generated) of 1.5, 2, 2.5, or 3. Since bus speeds can be 50, 60, or 66 MHz, the Pentium runs at 75 to 200 MHz internally.
The Cyrix 6x86 CPU sits in a Socket 7, too. It has a clock multiplier fixed at 2, so do not try any other jumper setting. Since bus speeds may be 50, 55, 60, 66, or 75 MHz, the 6x86 runs at 100 to 150 MHz internally.
In the basement of some companies, they may still have the old "P5" Pentium motherboards, with large 5 volt Pentium CPU's at either 60 or 66 MHz. Note that the old P5 boards cannot have a P54C style CPU, and vice versa. I usually use the generic P5 term to indicate Pentium and Pentium pin-compatible computers, including:
The Pentium-for-486 flavor chip is called the Pentium Overdrive, which is exactly what the upgrade chip for Pentium systems is called (to keep them separated, the ones for a 486 only have two speeds: 63 and 83 MHz). Cyrix and AMD use the exact same moniker as one another for competing chips, as in the 5x86 and 5x86. These sound like they're Pentium clones, but they go in a 486 motherboard.
The 486 is generally not for Windows OS's. A 100 MHz 486 may perform as well as a Pentium with DOS games and other programs. A faster CPU is not a replacement for too little RAM, however.
The Pentium Pro chip sells for less than the Pentium chip in some cases. The motherboard prices can be quite similar, too.
I prefer Intel chips for a number of reasons, but the main ones are that they support SMP and have great floating-point performance. The 5x86 is made by AMD and Cyrix for 486 motherboards.