It is 200Mhz faster per core and is the same architecture with the same cache the Phenom II X4 840 would basically be an Athlon II X4 640 at 3.2Ghz and it will work on the board you listed. For the extra money the Phenom II 840 would be a good deal compared to the Athlon II X4.
Which one is newer? Phenom II X4 840 or the Athlon II X4 640? I think I'm gonna buy the Phenom II X4 840. Same price, faster frequency, and I will upgrade to higher end cpu later (built-in graphics processor). This is just a test to AMD products since I NEVER use AMD before. Intel is good put the prices is toooo much.