Personally prebuilt systems are the worst thing, especially if you know what you are doing with computers. Prebuilt systems are expensive, non-upgradeable, and just down right suck. The fact that you cannot upgrade them means that in a few years it will go straight to the dumpster (all its worth in my opinion).
Here is my quick breakdown (off the top of my head) of what it would cost to build nice OC-able system yourself: (all in USD)
Case: 50-100 (decent case, 50 and you have to cut your own holes for fans, 100 you dont)
PSU: 40-120 (if not included in case, price depends on your tastes)
Motherboard: 100-200 (Higher price usually more options for OCing)
CPU: 100-200
CPU Fan: 40-80 (I paid 40 for mine and it works great)
RAM: 100-300 (Depending on quality and amount(1-2GB))
Video Card: 100-200 (Prolly gotta get a sale to get a good deal)
Hard Drive: 100-150 (300GB prolly gotta get a sale here also)
Floppy Drive: 5-7 (these are super cheap if you dont have one laying around)
DVD/CD Burner: 50-100 (again, might have to get a sale)
Sound Card: 25-50 (Mine is a 5.1 Sound Blaster and is only 25-35 these days, works great)
Speakers: 70-100
Fans and Fan Guards: 55-65 (If computer case needs to be moded)
These are prices that i have paid myself.
So the total comes to 835-1672
This is of course asuming you have a keyboard, mouse, and monitor. If you do not add the following:
5-60 for keyboard OR 20-60 for combo (even wireless)
10-50 for mouse OR 20-60 for combo (even wireless)
Monitor: 120-300 (CRT-LCD monitors)
Note, these prices DO NOT include rebates which are very common. ALSO, if you decide you have to go all out on components which is not nessicary it could cost considerably more. (For example: 150 on a wireless keyboard and mouse that is really no better than one that costs 60)