Trouble is, you can play this game endlessly - there will always be something cheaper tomorrow!
If you are comfortable at 1080 now I'd wait until you get your new monitor before looking at a new GPU - for one thing you could end up going down FreeSync/G-Sync route and might need a specific card to get the best out of it!
For my part I
think I've decided on the 290X but I'm just struggling to decide between the
MSI Radeon R9 290X LE Gaming Edition and the
Powercolor Radeon R9 290X PCS+ OC for the supposedly superior cooling for an extra £20!?!?!
In reality I'm unlikely to overclock the GPU so is the better cooling worth it if I'm leaving everything at stock??