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Having built 50+ custom made PC's (used to work for a small PC Shop), there are a few things:
1. Get the best monitor you can. CRT Monitors generally have a life of 2-3 boxes. (mine is 7yrs old and still looks as good as the day I bought it).
2. Get a high quality case (Aopen, Antec, Macase are all good). (read Anandtech for reviews).
3. Get a high quality power supply (400W+ needed in modern PC's).
4. Get surge protection or a UPS, (think of at as your insurance policy, on all the nice new shiny equipment).
5. Buy the current generation motherboard, (i865/875 or nForce2 Ultra 400). (provides scalability). And go with a well known brand, Asus (single CPU), Tyan (on Duals).
6. The rest will fall into place...
PS. Get a good keyboard and mouse (current Logitech or MS are good).