Buying a New Computer ... Help!

Dear Friends and Countrymen,

The time is nigh for Harrington to buy a new computer, alas there is one small problem; I have no idea what I'm doing.

I want a desktop computer (I already have a functioning laptop, in fact I'm using it to post this very message!), which will have two screens ("monitors"), be wifi-capable and will be able to run Sim City 4 without any lag - truly the bane of my existence!

I have never bought a computer before, so I have a few questions which I hope some of you will be able to provide advices on ...

• Where is a good place to purchase a new computer?
• Should I consider building my own computer? I have never done something like this before but a good friend (went to Eton) told me that it's an educating and interesting task
• What are good models of desk top computers? Have you owned one which was especially fast, reliable, etc?

Many thanks chaps,
Harrington