I have Blender v2.70 and can´t render with my GPU.
GPU: AMD Radeon HD 7800
CPU: i5 3450
GPU rendering only works with nVidia Graphics cards, it is specifically made to use their CUDA cores. You will have to use CPU rendering for ATI cards.
there is a thread about using open cl with ATI cards on here
have a look at that but I have no idea how well it works