1440 wise you could go 1070 £420 for FE or R9 390 or R9 390X the 390 is £
[email protected] and the 390X is £249.99. For RX 480 the Sapphire nitro is coming out in a couple of weeks priced at £249.99 but I would wait to see the reviews first to see if its 1440 capable.
I have a Sapphire R9 290X Vapor X 4GB and I game at 1440p with no issues in current games i.e. DOOM GTA5 etc
EDIT: Change that PSU ASAP OCZ are known for popping and taking your components with it