1.5. CPU (Central Processing Unit)
The CPU, the chunky chip serving as your computer's brain, determines how quickly your computer follows instructions. The more powerful the CPU, the faster it can fling around information, leading to less time waiting at the keyboard. Windows XP works well with CPU's from two companies: Intel's Pentium 4 (fastest) or Celeron (slower) CPU, or AMD's Athlon 64 ( fastest ) or Sempron (slower) CPU.
Engineers measure a CPU's "thinking" speed (known as clock cycles ) in megahertz (MHz) or gigahertz (GHz); the bigger the number, the faster the CPU, and the more you pay for it. This knowledge comes in handy when shopping for computers, as you know that a 3.6 GHz PC is slightly faster (and more expensive) than a 3.2 GHz. It's mostly gamers and movie editors who need super fast, super expensive PCs. For word processing and e-mail, most people do fine with a bargain-priced 2.4 GHz Celeron or Sempron.
Once you bring your computer home, you can't do much more with the CPU besides look at its perch on your motherboard. (And even peeking isn't easy, because the CPU's always covered with a fan or metal flippers to cool it down.) In theory, you can pop an old CPU off the motherboard and pop in a faster one, but it's rarely worth the effort, for several reasons.
Figure 1-4. Your motherboard contains your CPU, battery, memory, memory slots, cards, slots, and connectors for plugging in parts and cables. Motherboards vary slightly in their layouts, so your's probably looks somewhat different. But these main components appear on every motherboard. (The power supply connects to the case, not to the motherboard, but its wires lead to the motherboard to feed it power.)
First, replacing a CPU is expensive and time consuming. Second, different types of CPUs require different types of motherboardsthey're not nearly as interchangeable as most computer parts are. Finally, a PC's only as fast as its slowest part. That slow part is most likely the hard drive, the video card, the memory, or all three. In most cases, adding a slightly faster CPU to your motherboard won't make a noticeable speed difference. If you're itching to speed up your PC, try adding more memory (Section 1.6) or a faster hard drive (Section 9.2), instead.
When it's time to replace your CPU, it's usually time to replace the entire computer.