Most motherboards today retail for $200 to $300. Mid-speed processors cost from about $170 (for a Celeron 566 or Duron 700) to $480 (for a Pentium III 800 or Athlon 900). Don't skimp on RAM: go for 128MB (about $230).
For a total investment of between $600 and $1000, you'll have a powerhouse PC. If your budget is smaller, consider 64MB of RAM (about $100).
Most computers made in the past four years have cases that require a motherboard with an ATX form factor. If you're replacing an ATX motherboard, you can choose from a wide variety of boards, differing mainly in the processor types and speeds they support. Your best bet is to choose the processor you want and then purchase a motherboard that supports it.
If your PC is older, its case probably requires a board with an AT form factor. Check your system manual to be sure, but if your serial and parallel ports aren't built into the side of the board, you probably have an AT motherboard.
If so, consider buying a bare-bones ATX system - essentially a motherboard (usually with CPU and RAM) installed in a case with a power supply - and then transfer the drives and cards from your old system to it. Take care, however, if you have lots of ISA add-in boards: most new motherboards have few (or no) ISA slots.
The occasion of replacing your motherboard is also a good time to upgrade other components in your system, such as the hard drive or the graphics card.
Important: before you start your motherboard transplant, run a full system backup.