Acer is now selling AX1420-UR10P desktop for a pretty low price tag of 350 dollars. It is powered by AMD Athlon II X4 645 processor, which runs at 3.1GHz clock speed and comes with 2MB cache. Its TDP is 95 watts.
Specs include Nvidia GeForce 6150SE GPU, 4GB DDR3 RAM, 500GB 7200RPM hard disk, DVD writer and windows 7 home premium 64 bit operating system. Other features include gigabit Ethernet, 220W power supply, media reader, one PCI slot and one PCI-E x 1 slot, and a 1-year limited warranty.
While it is quite powerful, it is not recommended to be used as an HTPC unless you pickup a more powerful video card.
Other AMD processors worth checking: