CPU: AMD Athlon II X2 245 Regor 2.9GHz (very basic AMD core, it'll do what you need it to do)
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103687
$59.99
MOBO: GIGABYTE GA-MA785GM-US2H (looks good and steady, reviews are positive)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128394
$69.99
HDD: Western Digital Caviar Blue WD2500AAJS 250GB 7200 RPM (it's a western digital, good things)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136113
$42.99
MEM: CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (dual channel for better performance)