The fastest model being the former FX-60 for a 2.6ghz dual core model was unfortunately discontinued rather fast by AMD. For many gamers the fast single cored FX models like the FX-57(2.8ghz) worked well while a 4600+ or 4800 see a slight gain at 2.4ghz over a the single core 3500+'s 2.2ghz clock speed. But those are dual core better for any multitasking in that sense.
The 4000+ is still a single core model still only seeing the .2ghz increase. Here the last build run temporarily while upgrading a family member saw a 3500+. Once into the current AM2 build the 6000+ X2 saw a leap there at being dual core and 3ghz. For the 939 board a 4600+ or 4800+ for dual core or the higher FX55 or FX57 looks the best. You can look over the information on all 939 models at
http://products.amd.com/en-us/Deskto...2000&f10=False&