The GTX970 is a good choice, it won't just go beyond 300w even if overclocked or under stress test, unlike R9290, but both will need 6-pin & 8-pin pcie plug. Your PSU has +12v 44 Amp rail (total 528w on 12v rail) and it has 2 x 6+2-pin pcie, means your psu is sufficient for your card and system.
Though your psu can handle the setup, since you will be adding this high performance card to be added in your system (either R9 290 or GTX970), surely it will draw load higher than your recent setup, your psu may degrade over time then you'll know when its time to change psu. Besides, GTX970 has better performance, lower power consumption and cheaper than R9 290.. the bucks save may need to use on better psu later on..
GTX970 vs R9290...