I might suggest a Celeron just to cut down on price. Heres a Pentium D build though(didn't include price of case or OS)
CPU: Intel Pentium D 805 Smithfield 2.66GHz 2 x 1MB L2 Cache LGA 775- $93
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RAM: CORSAIR ValueSelect 512MB 240-Pin DDR2 SDRAM DDR2 667 (PC2 5300)- $53.99
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MOBO: Foxconn 945P7AA-8KS2 Socket T (LGA 775) Intel 945P ATX Intel Motherboard: Foxconn 945P7AA-8KS2 Socket T (LGA 775) Intel 945P ATX Intel Motherboard $77.99(67.99 AR)
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PSU: FSP Group (Fortron Source) ATX400-PA, version 2.0, SATA, 400W Power Supply-$36.99
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DVD Burner: Samsung 18X DVD Burner-$29.99
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Hard Drive: Seagate Barracuda 7200.9 ST380811AS 80GB 7200 RPM 8MB Cache SATA 3.0Gb/s Hard Drive-$44.99
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Total Price(without shipping)-$336.95
Just add in a case and an OS and you're set! I'll do an AMD build also just to see which one is cheaper. Athlon 64 or Sempron?