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Asus does not show that processor as being supported by that motherboard, strangely, it stops at the 955 x4 and skips to the Phenom x6's seen here (gotta scroll way down - if it's any consolation, my AthlonII 455 x3 isn't listed either) http://www.asus.com/Motherboards/AMD_AM2Plus/M2N68AM/#C...
Means they probably do not have a BIOS version that will work with it. I think you're out of luck with that one.
It will physically fit, that isn't the problem, some mobo makers have AM2+ BIOS versions which will even work with that processor, so that isn't really a problem, except, it appears Asus does not have a BIOS version for that particular motherboard/CPU combination - that has the potential to be a big problem.