You are wrong, AM3 CPUs do work in old AM2 boards. They are usually compatible with a bios upgrade. Sometimes the bios updates are not listed on the manufacturer's website but are released. In your case there is a bios update for AM2NF6G-VSTA. It is Bios 2.39 and you won't find it on AsRock's website. Instead you can find the bios update on www.pctreiber.net. It's a German site. All of the Athlon II and Phenom II Processors should work if they are 95 watts except for the X6. Bear in mind that the HT speed will be cut down because AM2 boards only support up to HTT 1.0 but that will have little impact on your overall performance.
Baseline Benchmarks gathered from passmark Performance test 7.0
#1 Athlon II X4 630@ 2.8GHz scored 3546 CPUMARKS running on HTT 1.0
#2 Athlon II X4 630@ 2.8GHz scored 3595 CPUMARKS running on HTT 3.0
In the Benchmarks #1 had 4GB of 667MHz DDR2 RAM on an N68S3+ motherboard
In the Benchmarks #2 had 4GB of 1333MHz DDR3 RAM on a GA-MA78LMT-US2H motherboard.
Both systems were running Windows 7 64-bit.
As you can see, HTT 1.0 to HTT 3.0 has little effect on the system performance and it is consistent on systems running the same software with the same amount of RAM and such. Hopefully this will shed some light on the subject for other people who are interested.