these reviews may prove helpful. they certainly helped me decide what to get.
and the x2 4200 and x2 4400 both have 2.2ghz. only the x2 4600 and x2 4800 have 2.4 clock speeds.
the difference is simply the cache size. and a smaller cache size does have some benefits. this is taken from the first link:
"Since the L2 cache has been physically reduced on the Manchester core, this drops the transistor count from a whopping 233 million, down to only 154 million. It also results in a smaller die size of 147mm2, compared to 199mm2 for the Toledo. You would think this would also reduce the thermal power requirement, but it remains unchanged at 110W for the Athlon 64 X2 4800+/4600+/4400+, while dropping to 89W for the 4200+. This lower thermal specification makes the Athlon 64 X2 4200+ an especially attractive upgrade option for existing AMD systems, as it means fewer potential cooling issues, and less stress on older platforms".