The power supply has two rails, therefore the +12V1 which has 18A or 18Ax12V=216W wil be powering your mainboard CPU and PCI-E slots. Based on your spec at i7 4770 at 84W TDP, 75W on our PCI-E 16x plus any additional component in your system such DVD-ROM etc you are just getting by. Now your second rail +12V2 has 17A or 17Ax12A=204W which will be powering for example the PCI-E power connectors etc. The R9 290 has a TDP of 250W to 275W, therefore it will not be sufficient with the current power supply you have.
If the power supply was a single rail design you might be able get by but we strongly suggest to get a more powerful power supply. If you choose a multiple rail power supply please make sure that you have sufficient Amp to power your components.