Intel Celeron 1.7GHz

128 MB RAM

Integrated Intel or 32MB nVidia TNT2 class graphics processor

Integrated Sound

48X CD-ROM

40GB Hard Drive

17 Inch Monitor

This system would suit your basic office. System uses would include the using the Internet, Word Processing - Microsoft Office programs. Also any other basic programs. 

Intel Pentium 4 2.4 GHz

256MB RAM

nVidia Geforce3 Ti200 128MB Display Card

Integrated or Creative Sound

48X CD-ROM and CR-RW Drive

120GB Hard Drive

19 Inch CRT or 17 Inch LCD Monitor

This system would be great for the office where power to run application is needed. Not only would this system run basic office applications, but also 3D graphic designing, and programs that need lots of memory and horsepower.

Intel Celeron 1200MHz

128MB RAM

Integrated Intel or TNT class graphics processor

Integrated Audio

48X CD-ROM

40GB Hard Drive

17 Inch Monitor

This PC would suit the first PC buyer or a guy on a budget. Great to do the basics on - the Internet, Schoolwork. Games would be on the downside - they would run, but not with the best image quality.

Intel Pentium 4 2.4GHz

256MB RAM

128MB nVidia GeForce4 Ti4200 Graphics Card

Creative Sound

48X CD-ROM or 16X DVD-ROM and/or CD-RW Drive

80GB Hard Drive

17/19 Inch Monitor

This PC would do all programs and applications very nicely. From Word Processing right through to games like Unreal Tournament. Expect at least 50 FPS. Has all the guts to run all things with no problems.

Intel Pentium 4 2.8GHz

512MB RAM

128MB nVidia GeForce4 Ti4600 Video Card

Creative Sound

16X DVD-ROM & CD-RW Recorder

120GB Hard Drive

19 Inch CRT or 17/18 Inch LCD Monitor

If you want the power to do everything but fly to the moon and back, this is it. This is suitable for the 'hard core gamer' who wants the best performance and top image quality out of his games. Also, this system has ample power to run Office applications, and intensive programs.

1. Look for good quality Computers. A supplier that promises technical support does not always mean it will be a good buy. A PC that never breaks down with no technical support included is better then a PC that always breaks down and always going to get repaired. 
2. There's nothing wrong if you buy from a private person. If you get quality, and a stable system, that's good. Also, it's great if you get what you asked for.
3. If the computer you want to buy is available to look at, ask to have its case removed to see how well it has been assembled. Untidy cabling usually indicates a sloppy assembly, for example. Of corse, you can't do this if the computer is to be built.
4. Arm yourself with third-party support. Join a PC or software group user group to both give and receive help and advice without paying for it. Bookmark all the web sites that are relevant to your computer or software.

Just another point: Don't sweat the small stuff. PC are like any other appliances. If using your computer is very important to you, it might be wise to have a spare around, just in case the other one falters.