Personally I believe that brand computers are not as good as a computer built by yourself. The reason for that is because the engineers that design custom computers are always trying to find cheaper components to get more profit from you!

That is not the case of Laptops and this guide will not help you if you require a laptop.

The first thing you need to decide is what kind of processor you want. This guide will use AMD processors because I consider them better than Intel processors.  My opinion is based on several hardware reviews, but the most critical review comes from gamers.

Visit any serious gamer review page and you will end up buying AMD. So, after being a serious gamer for more than a decade, I can say that unless you buy everthing Intel, its better to have AMD on your side. By everthing I mean Motherboard, video graphics card, sound card, etc.

a)The first thing to decide is the processor. You should decide which processor do you want. And this decision should come from your wallet, not you. What are you willing to pay?

My recommendation is a Dual Core AMD 64 bit processor or AMD x2.

b)After picking up the processor, spend some time in the AMD web page deciding which mother board is the best for such AMD processor.
Any motherboard coming from that page is good. My personal choice is MSI and ASUS, but thats me.

c) Choose based on the mother board you picked the memory to be used. Some motherboard brands actually recommend the brand and model to be used. Dont trust your regular brand of memory, there is nothing written on memories and only the motherboard manufacturer can actually recommend them.

d) Ok, you have CPU, Mother and Memory. Its time for Power Supply. Dont be fooled by the power output of the power supply brands. What you need is the voltage that your cpu receives. Generally if that is ok, then everthing is ok. Check this out and you will never see the horrible typical blue screen of power supply shortages. Some power supplies cost less than the shipping charges for it. Do you want to trust your CPU to those cheap names? Ultra power supplies are my personal choice.

d) Get some SATA disk for Hard drive. IDE is in the past. Currently the speed rpm is not such a big deal. What you should look for is the most possible buffer size available. Thats more important than RPM's or than disk size.

e) Get a nice case. Thermaltake is the best. Your wallet is the one that makes the decision this time as well. Decide if you are going with several fans or with water cooled systems.

f) Graphics is a question of mental sickness. Do you really want to have the best available graphics in the market? Then be ready to pay for them. With the new motherboards, you can actually join two PCI express cards and have all the best graphics work as fast as a NASA computer. Dont look for RAM. Ram is what the manufacturers use to get noobs for their cheap end video cards. Get something that gamers approve. Gamespot and other descent websites should be the ones for that. 
If you are not gaming, get whatever is out there that is compatible with your motherboard.

g)See my guide for monitors.

If you did your homework ok until this point, the accesories you use depend on your special needs. For example, if you use video, you should focus on a video capture video card and the latest 1340 port drivers in the market.
All the motherboards have 2.0 USB connections. If your motherboard does not, then you are not doing your homework correctly.
Up to this point the same computer that DELL sells , you should be able to get it about 20% less expensive.
Its really worth the effort.

Be safe