Mobo: Asus M4A77D AM3/AM2+/AM2
$70 after rebate
Optical: Get any cheap SATA DVD burner. They're usually about $25.
PSU: Corsair 550W
$85 after rebate
GPU: HD 5770
$150 after rebate
HSF: Coolermaster Hyper 212 Plus $30, if you just want to replace the heatsink as well...
Total: $360. This build is a good base to build on. The board will accept the newer AM3 CPUs, while the GPU is a newer DirectX 11 card that will be more future proof than any nVidia card. The PSU is a good quality unit that will be able to power some of the more powerful GPUs if they were upgraded.