In general the Athlon will be slightly slower clock for clock than the Core2. Being it's clocked over a gigahertz faster that more than makes up for it though. I would go with the x3 455.
Some games do better with quads but a higher clocked triple core is just as good or better in 99% of any situations I can think of.
You also have the option of at least trying to unlock the fourth core and some cache of the Athlon.