I agree with Checkmate statement and you have so many possibilities, a lot of manufacturers and brands. You have not written what are your requirements and application, but to help you in general lines:
1) Choose the processor: Intel, AMD, Via, PowerPC
Check in the manufaturers sites to see the models and specifications of each processor. The clock is not a parameter to compare performance. AMDs processors with lower clocks than Intel processors can have the same or better performance. Check for the heatsink+cooler problem, as with high speed clocks, the processor can burn literally if not properly cooled.
2) Memory: RAM size, type of module and speed (eg. 256Mbytes DDR 400 PC3200). The speed must match the FSB speed.
3) Motherboard: size, number of slots and types (PCI, AGP8X), on board peripherals (video, USB ports, parallel and serial ports, IDE interfaces SATA, sound, etc), slots for memory modules, Front Side Bus clock (e.g 400MHz). The motherboard must support the processor you have choosen (and the cooler). Brands are very important, don´t buy very cheap MB.
4) Video board: AGP8X bus, size of memory, Nvidia or Radeon types, with or without TV out, Video capture, etc
5) Hard Disk size and speed (10000RPM), interface type (SATA)
You can look in the manufacturers sites and check the manuals and specifications sheets if they match your requirements for a new computer.