In terms of AMD, AM2 is definetly the way to go. Socket 939 can transfer data up to around 7 gigabytes/s, but AM2 on the other hand transfers data up to 11 gigabytes/s! This also depends on the type of memory though. Basically the faster the memory the less overall performance boost you will see from 939 to AM2.
Yes it is true there have been memory problems with the AM2. This is due to poor memory controller coding in the chip itself. So make sure you do your research on whether your memory will be affected or not.
Let me know if this helped.