The GPU is the weakest spot in your rig, however depends what games you play and at what res. Is the 7950 struggling at anything?
Difference between 290 and 290X is negligible
YOUR BASKET
1 x Powercolor Radeon R9 290 PCS+ OC 4096MB GDDR5 PCI-Express Graphics Card £319.99
Total : £319.99 (includes shipping : ).
What monitor do you have?
Don't buy a 4gb gpu. 2gb is better
What?That's a false statement, no idea how 2GB is better than 4GB
Maybe selling those monitors and buying a 27" one is an option?
Any reason you're going with Nvidia? Do the games you play utilise physx or do you just fancy a change?
Yes I would be going with 4GB at least so you can take a look at the following GTX 780 which has 6GB VRAM
http://www.overclockers.co.uk/showproduct.php?prodid=GX-191-GW&groupid=701&catid=1914&subcat=1341