Check this website for all compatible socket 754 processors:
http://products.amd.com/en-ca/desktopcpuresult.aspx?f1=&f2=&f3=&f4=&f5=754&f6=&f7=&f8=&f9=1600&f10=False&f11=False&f12=False
You'll want to be sure your motherboard/laptop BIOS will support the CPU you select. You should be able to check the manufacturers website for this information.
It looks like the highest performing CPU is the AMD Socket 754 3700+. The processors are running around $70 on eBay.