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A number of different upgrades can boost your computer's performance, but for turbocharging your old PC, nothing beats a full motherboard transplant. A new motherboard, coupled with a high-speed processor and a generous amount of RAM, can dramatically improve system performance. It's not a job for a beginner, but if you're comfortable doing major poking around in your PC (or have a computer-savvy friend to help), the operation isn't too difficult.

Most motherboards today retail for $100 to $150. Midspeed processors, meanwhile, cost from about $100 (for a Celeron-500) to $200 (for a Pentium III-600 or Athlon-700). Don't skimp on RAM, either. Go for 128MB (about $140). For a total investment of between $350 and $500, you'll have a powerhouse PC. If your budget is smaller, consider 64MB of RAM (about $70) and a lower-end CPU like AMD's K6-2-500 (about $60), dropping the bottom line to about $300.

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. Watch out, 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. Before you start your motherboard transplant, of course, run a full system backup.