Processor Intel® Core i5-4690K CPU @ 3.50GHz Manufacturer Intel Speed 3.5 GHz Number of Cores 4 Video Card 1 ASUS R9 270X Series Manufacturer ATI Chipset ASUS R9 270X Series Dedicated Memory 4096 MB Total Memory 4.0 GB Video Card 2 Intel® HD Graphics 4600 Manufacturer Intel Chipset Intel® HD Graphics 4600 Dedicated Memory 192.0 MB Total Memory 1.8 GB Memory 16 GB Operating System Microsoft Windows 7 Professional Edition Service Pack 1 (build 7601), 64-bit Service Pack 1 Size 64 Bit Edition Professional PSU 750watts 80+ gold

the specs above show what is inside the computer, i also have a cpu cooler from cooler master.
the plan is to overclock the cpu and also to get a new/better GPU but im not sure what the GPU would be best for my system. my budget is around £300 but don't mind going over a little.

side notes the maximum length for the gpu can only be 32 cm as my case is fairly small.

any help would be greatly appreciated :)