How much graphic card memory I need to run minecraft shaders with the realistic clouds?

Do you mean the Sonc Ether's Unbelievable shaders mod, you're going to need a lot. You also need a good CPU, since Minecraft itself mostly relies on CPU power. I currently have an i7-4770K @ 3.5 GHz and a 4GB GTX 760, and run it at ~45 fps on 1x shader res and 30 fps at 4x shaders res. I also once used a MacBook Pro with an i5 at 2.4 GHz, with 384MB VRAM, and got about 8 fps.