I'm not going to link you, because what I link you to might change price. However, I will tell you want to look for.
Processor - Go with the Core 2 Duo (or Quad, if you can find one in your price range) in the 6000 series with a clock speed of at least 2.5 GHz. These processors will be faster than their 4000 series counterparts because of more cache RAM for the pair of processing cores.
Motherboard - Intel 965 or better with at least one PCI-E x16 slot. Crossfire/SLI support is also a good thing.
RAM - 1 GB minimum, 2 GB is more realistic.
Video - nVidia will usually give you more performance per dollar than ATI will. For nVidia cards, go with a 7600, 7800, 7900, 8600, or 8800. GS is fast, GT is faster, GTX and Ultra cards are fastest. For ATI cards, go with a X1600, x1900, x2600, or x2900. More letters at the end of the card name usually mean a faster card. Also, get at least 256 MB of video RAM, but remember that more is better.
Drives - 160 GB SATA hard drive or larger, DVD/CD drive that can read and write all. If you get a GeForce 8600 or a Radeon x2600 or x2900, Blu-Ray or HD-DVD are also viable choices, though easily as much for the drive as what you want to spend on the computer.
Other stuff - High output power supplies are inexpensive and worth the investment over something cheap and underpowered. Most sound cards aren't any better than integrated audio, even if they claim they can accelerate gaming (that claim is usually unfounded). Gigabit ethernet isn't faster than 100 megabit ethernet on a standard 10/100 network or over a broadband connection to the internet at this time, but makes sense if you go to or host LAN parties; you'll need a gigabit switch, hub, or router, and Cat 5e or Cat 6 cable to support gigabit ethernet.