If you are using 4GB of RAM, then you need to make sure you are using a 64bit OS. 32bit OS's are only compatible with 3GB of RAM or less. You also need to make sure your motherboard has enough slots for the RAM sticks. If your mobo only has 2 slots, then you need to go with 2x2GB sticks, if it has 4 slots, I would still recommend 2x2GB sticks incase you may want to upgrade at a later date, however 4x1GB sticks might be a cheaper option.
Another thing to consider is case and GPU compatibility. Some GPU (Video Cards) will not be able to fit in some smaller cases, this is why, as far as cases go, cheaper is not always the best way to go. I am assuming you will either be going with a GeForce 8800 or 9600. I would recommend an 8800, but again, make sure it will fit in the case you get.
Also, make sure your Mobo and CPU are compatible, don't get a AMD Mobo and an Intel chip, and if you are going fully AMD, don't get a AM2 and a quadcore, etc.
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Oh, and I'm not a mod anymore, so I couldn't close it even if I wanted to.