how to buy computer?

Features
The key components of a computer are the processor, memory, operating system, hard drive, graphics adapter (with video RAM), optical drive, and display (monitor). Laptop computers have additional features and considerations that are important. Where applicable, we've noted feature information that is important and distinctive to the type of computers.

Processor | Random access memory (RAM) | Operating system | Graphics adapter and video RAM | Hard drive | Optical drive | Monitor | Display | Case | Battery | Mouse | Touchpad | Keyboard | Sound system | Ports | Card slots | Docking station | Log-on security
Processor
This is the computer's "brains." Clock speed, measured in gigahertz (GHz), and the chip's design, termed "architecture," determine how quickly it can process information. Within a processor family, the higher the clock speed, the faster the computer. But different processor families reach different efficiencies.

For laptops: Laptops generally come with a dual-core processor. If you're on a budget, an Intel Pentium Dual-Core or AMD Turion 64 X2 is fine. For greater power or battery life, get an Intel Core 2 Duo
For desktops: The lowest-priced Windows systems probably use Pentium Dual-Core, Celeron D, Athlon 64, or Sempron processors. But most common now are dual-processor desktops. Dual-core processor families from Intel (Core 2 Duo) and AMD (Athlon 64 X2) represent newer technologies developed to increase processing power beyond what a single-chip processor can achieve. Macs have transitioned to Intel Core 2 Duo series processors. Quad-core processors are also becoming more common in higher-end desktops, and AMD also offers a triple-core processor.

The different processor families make direct speed comparisons difficult, but any recent processor family will probably deliver all the speed you need.