How about running the R9 290 with energy savings mode on? Would that be better than the 660?
Energy savings will not be effective when you are doing actual gaming.. During gaming, the components will consume a lot of power as it has to process a lot of data. and there is a shortage of power with your current PSU. So if there is a shortage of power, the PSU will not be able to cater the needs of the power and it will fail. The PSU when failing, has the potential to destroy your other components along with it.
Hence, it is not advisable to use the GPU with that PSU.