Are you unable to view the cpu compatibility chart for your mobo on Asus website?
http://support.asus.com/cpusupport/list.aspx?SLanguage=en&p=1&s=22&m=P5G41T-M%20LX3&os=&hashedid=pi8H2FaVJacvtnN6
G41 is an Intel chipset, you cannot use AMD cpu's on an Intel chipset, you can only use Intel cpu's.
However those cpu's were discontinued years ago, your best bet is to find one used on ebay. They are usually very expensive though.
Newegg does have a few refurbished cpu's that will fit your mobo, but only dual cores. Here's the fastest one I can find, which is pretty pathetic in my opinion.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116424
You can get a new modern motherboard/8GB DDR3/and a modern cpu for around $175-200.