The 7850 is faster in many benchmarks, but the 7790 beats it in a few due to newer architecture and higher clock speeds. The 7790 has a TDP of 85W compared to 130W for the 7850 so that might be a big factor when considering your power supply.
thanks,looks like ill go with the 7790 due to my low psu wattage. the 7790 dosnt look to bad with the benchmarks ive checked and will be much better than what i have now. thanks again