Well the main difference between the Propus (Athlon x4) and the Deneb (PhenomII x4) CPUs is there target audience. AMD targeted power users and people looking to get into quads with the Propus, so they most likely don't OC as well as the Denebs and they don't have L3 cache or as high of clocks. If you want to get 'beefiest' CPU from AMD, then get either a C3 stepping 955 or 965 PhenomII x4's. They're around $175 - $195 on NewEgg. You can tell they're C3's (which just came out) becuase they'll have an "GM" instead of "GI" in their name. C3 stepping PhenomII x4 955 on NewEgg: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103808
However, the Propus are still an amazing deal for only about $100 for the 2.6ghz and $115 for 2.8ghz. They just wont perform as well as a Deneb.