The Intel system mentioned above is pretty good, but you can save a ton of money without sacrificing too much performance by going with AMD.
If you plan on overclocking, the Athlon X2 BE-2350 (about $100) should be able to get about a 50% overclock (the X2 BE 2300 is cheaper and should also overclock well, but I haven't seen any reports of it yet). If not, the X2 4600 (65W) is also about $100, while the 4200 (65W) and 3800 (65W) both retail for significantly less ($80 and $65, respectively, on NewEgg). The Biostar TForce 550 is a great overclocking board, and rings in at $80. A combo with these would run between $150-180, which leaves an extra $170-200 to keep or upgrade other components with.