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I suggest if you already have a reasonable PC you should mess around and upgrade some components, such as the hardrive or install a CD burner yourself before tackling a PC from scratch.
It is better to get someone to assemble a PC for you unless you know what you are doing as you can easily damage expensive components with static or rough handling. Also if you do it yourself you will need to buy cable ties, an antistatic wrist strap, screws etc which will probably be more expensive than the labour.
Also DIY projects are often plagued with incompatible drivers, poor hardware doucumentation and other glitches which can be very frustrating for newbies.
Make sure you know exactly what hardware you need before you start. Combining a 1.8GHz Athlon with an ATI 9800 Pro is a mismatch for example.
Hold off 3-6 months so that new hardware such as SATA RAID and 800MHz buses become mainstream or you will be stuck with limited upgrade options in future.