Reposting this again:
4000 low, 2600 high. $600. Does not include HD, DVD-ROM, OS, Case. If you already own a computer and have the OS/HD/DVD/Case, then you can buy my system at that price. If you're trying to build one from scratch, HD is about $55-60, DVD-ROM is $20-30, OS is about $130 if you buy OEM, Case is $30-100 or more (an average one will run about $50-60ish; I paid $90 for mine).
Intel rigs (Core i5/i7) will set you back a little more, and the i7 will provide much higher performance than the Phenom II x4. The Ph II x6 will be better as well, but it is more expensive. Similarly, an nVidia GTX Fermi or a Radeon 58xx will also cost more and provide better performance.
Don't misunderstand: There is no such thing as an inexpensive, top of the line rig. You're either cutting down on performance to save money or you're spending money to get performance. I believe that for $550-850, you can make smart buying choices to get a GOOD system for a GOOD price, but always bear in mind that spending more means getting more and spending less means getting less.
If you can't build it yourself, expect to pay more money for the same thing. A $1500 retail system would cost around $900-1200 in parts to build the same or better. A $1000 retail system could probably be built (again, eitehr comparable or better) for $750ish. And as mentioned, if you already have some basic parts (HD, optical drive, OS, case) to work with, then you need to upgrade your GPU/CPU/MB/PSU/RAM. The less you have to buy, the more you can spend on it.
For all the people who say "But a PS3 only costs $300! A gaming machine will cost more than twice that!" They're right. A good (or better) gaming rig WILL cost at least $600 in total. If you're only concerned with "how much am I spending?" then a PS3 may be the better move for you. but if you have decided you want a good (or better) gaming rig, then you should set realistic price expectations. More is better, but too little will hurt you, and need to be upgraded sooner.