The 290 is designed to go head to head with the GTX 780. It delivers quite the performance advantage, you may want to check the review on it's performance here on Tom's to see some concrete numbers on it.
@1080p gtx 770 is enough. However r9 290 for £140 more is really worth it.
Either it is a win/win situation.
Save money then get an SSD/Cooler to OC your CPU.
If you already have an SSD and Cooler go get the r9 290.