Which R9 series are you upgrading to? Just for reference for what PSU you will need there is a list of R9 series cards and there PSU requirements. Also dont go cheap on the PSU get a certified (bronze or better) PSU.
Radeon R9-290X - 33A and a 600W psu minimum
Radeon R9-290 - 31A and a 550W psu minimum
Radeon R9-280X - 30A and a 550W psu minimum
Radeon R9-270X - 24A and a 500W psu minimum
Radeon R9-260X - 19A and a 450W psu minimum