it's my personal opinion , so there's no offense
as far as I've studied about this case there should be as much as 5-10% performance gain from AM2+ compared to AM2 (including DDR2-1066 in to account).
but the point is that , in Phenom Processors the memory controller supposed to run at higher frequencies than the processor but it's not happening right now , I mean at least in first generation phenoms (B2 Stepping I mean) , and the memory controller is running in a lower frequency than the processor so theoretically there should be less than 5% performance gain from AM2+ but you should also add this to the fact that the mobos based on amd 7th generation chipsets are faster than those from the 6th generation , I've seen a comparison between two of them , the most significant improvements were in games .
so if you don't have any other choices , don't bother
plus you should pay about 100$ more for the 790Fx or about 50 dollars more for the 790X compared to mighty 690G (which you can find it as low as 50 USD) so the performance gain doesn't worth the price , you can get 9600 instead of 9500 with that kind of money .
btw i thought that I've seen some sort of benchmark which have been compared 690 against 790 with Phenom processor , I'll just search for it and I'll let you know if i found it