The ECS MCP61PM-AM is an OEM (Gateway, Acer...etc) board, so the BIOS isn't kept up to date very well.
The best processor supported on that board is the Athlon 64 X2 6000+. Specifically the 89W model.
The 2nd best & 3rd best are the 5600+ & 5400+ respectively. (Also 89W)
AM2 processors aren't widely sold new in stores anymore (if at all), so you'll likely have to buy used. You should be able to find AM2 processors for well under $150 used though. Heck, it's more likely you'll find them under $50.