Good base system so no problems with the CPU restricting an upgraded card.
Few points:
The powersupply needs to be strong, 700Watts from a good maker with dual 8 pin PCI-E leads to be certain it can handle the upgraded card.
Some of the R9 290s are very large, the Sapphire R9 290 Tri-X is a little over 30cm long, so be very certain the card you decide on fits your system case!
The AMD options in particular demand good case ventilation for their coolers to work effectively, if the system case is small or only has a couple of fan locations you might be better off looking at a reference cooled (single fan, exhausts its heat out of the case) GTX780:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-068-KF&groupid=701&catid=1914&subcat=1341
There's not much performance difference between the R9 290 and GTX780.
You'll easily find a R9 290 under £300, delivery included, but both the R9 290X (not really recommended for reasons already stated) and GTX780 will exceed your budget.
A few nice places to look: Dabs, Novatech, CCL, Scan, Ebuyer, Pixmania, Overclockers, Aria and the omnipotent Amazon.
Top picks: Sapphire Tri-X, Sapphire Vapor-X, MSI Gaming, Asus DirectCUII R9 290.