Well, AM2/AM2+ has 940 pins, while current AM3 CPUs have 938 pins. The AM3 standard itself accepts 941 pins, suggesting that future AM3 CPUs will be AM3 only and physically not fit into AM2/AM2+ boards. Anyway, as far as the coolers go, 939/AM2/AM2+/AM3 all use a similar mounting bracket. If a cooler fits socket 939, it will fit on AM3.
As far as the best cooler, you may want to look up reviews on places like frosty tech as they just focus on coolershttp://www.frostytech.com/
You can also check out X-bit labs as they do some cooler round ups every now and then.