AMD Phenom II X4 925 Deneb 2.8GHz Socket AM3 95W Quad-Core Processor Model HDX925WFGIBOX - Retail
These prices are from www.newegg.com most likely the cheapest you can find unless you live near a microcenter.
Most likely you can reuse the power supply and your hard drive and that case you have. The power supply in the one you have now is a 20pin connector it will work on 24 pin mother boards if you plug it directly in the center of the connector on the motherboard. You can also buy an adapter to get you by until you can get a real power supply.