Depends on what kind of games you like to play. Are you one who is only fascinated with graphics and wants to have bleeding-edge gear to play games like Crysis? or are you wanting to just player Half-Life 2, World of Warcraft, and the like at good settings without breaking the bank and the environment?
For my Green PC, I built one that drains at max load, LESS than 200w, plus it is using modern parts and capable of running Crysis on low/mid settings with no problem (in XP). This may be overkill for some games being sold still. This includes a decent monitor in the setup as well.
Case: AeroCool M40 - Black - microATX cube case - holds the smallest desktop motherboards and biggest of graphics cards
Motherboard: GIGABYTE GA-G31M-S2L - microATX motherboard
Graphics Card: EVGA 8800GS, model 384-P3-N851-AR
Processor: Intel Core2Duo E6600 @ 2.4GHZ - low consumption, powerful processor
RAM: GeIL 2GB (2x1GB set) DDR2 800 - decent ram for games like Half-Life 2, WoW, Guild Wars, etc.
Hard Drive: Seagate Barracuda 7200.10 160GB - this is a single platter drive, resulting in a smaller, cooler, and more energy efficient hard drive, 160GB should be plenty for anything you throw at it (unless you plan on pirating stuff, which is a big no-no). For a bigger capacity, get 2.
Optical Drive: Lite-On DVD+/-R Burner SATA model LH-20A1S - this is one dirt cheap, reliable, and mostly quiet burner that drains little power.
Monitor: 19" LG L1933TR-SF monitor - this monitor consumes only 33w while in use, has a 2ms response time and costs right at $200.00. remember, only buy a monitor only if you really need one.
Cooling: 3x Rexus 80mm Panaflo case fans, ZEROtherm CF800 92mm CPU cooler, Arctic Silver 5 Thermal Compound (drops CPU temps 5-10*C compared to stock compound included with cooler)
Speakers/Headset: anything you want really, I got a pair of Sennheiser HD 485 for a good price and I highly recommend them.
Power Supply: Enermax MODU82+ 425w - this is a modular supply with removable cables, making cable management a snap. Also, this is one of the highest rated PSU's in efficiency, ~82-88%.
Keyboard/Mouse: Logitech Wave ergonomic keyboard, Logitech MX518 mouse
Cost: ~$950-$1200 depending on prices on the net.
Power consumption: ~130w at idle, ~195w at full use
OS: Ubuntu 7.10 w/ Wine 0.9.55
I know, Ubuntu, it's Linux and something else you probably never heard of. Well, first off, I want to let you know it's free. Second, Cannonical (the company behind Ubuntu) is behind giving free software to children all around the world, and has made a huge impact in Africa and is also pushing for a cleaner world (as are most Linux developers). Also, Ubuntu, with the help of Wine (think of Wine as a translator between Windows programs and the Linux OS) is able to run many games, and is the setup I use to play on my PC. Games I have running on this computer that are "Windows only" include Half-Life 2, Half-Life Episode 1&2, WoW, Guild Wars, Call of Duty 4: Modern Warfare, just to name a few. I also recommend the use of Linux if you have no remaining Windows XP licenses available as I would rather not have another person pirating software just to play games, its just bad business. Plus if you decide to use Linux, 99.8% of the software available to that OS is free, legally, plus I'll personally send you a copy of Ubuntu 7.10 for free if you'd like to try it out.
If you want to contact me about more info about building a system or anything else, you can email me at the-eternal-dark@(no spam)hotmail.com, remove the no spam part, just let me know who you are in the subject line and I'll try to respond quickly before the end of this week (April 6th I ship out for the Navy). I'm sorry in advance that I can't promise give help further in the near distant future, but basic only lasts 9 weeks, so you can contact me after that time period as well.
If you decide to buy from a manufacturer, I recommend Dell. The model I recommend is the Inspiron 530 with a dual core processor (either the pentium dual core or the core 2 duo), XP Pro, 1-2GB ram, Nvidia 8300 GS, and they will even plant a tree for you for an additional $6, totalling you about $695 without a monitor, $885 with the cheapest 19" Dell monitor. it is capable of running Half-Life 2 games at medium/high settings, but don't expect stellar performance. This is less powerful with equal power requirements and a little bit cheaper, so it's really up to you what you want to do.