I am applying an earth texture on to a sphere and I have resized the sphere so that it is quite large compared to the default size. I have tried some high res textures from that nasa page and they get a little better the higher the res but they still arent very smooth looking. what can I do to improve this?

If you mean pixelated, double the image size in photoshop (it will get rather pixely but don't worry) then take the gausian blur tool and blur it up untill it looks smooth at that rez.. If it doesn't have to be in perfect focus, this will work, If you want perfect detail... I am afraid you will have to call them to get the gigabyte image.

If you are trying to get a "close-up" shot of the sphere, the resolution of the earth map has to be higher than that of the render image, or there will always be pixels showing.

Actually it has to be a lot higher, you see at the top and bottom of the sphere, you could get it unpixelated with a 64 by 32 image, however near the equator the image must be equal to the circumferance of the circle in relation to the distance from the sphere, IE if you are rendering a sphere with a radius of 10 the camera would have to be 1/2PI10*10 away from the sphere for the image to be unpixelated. Because the center of the image gets stretched on a sphere while the top and bottom get...pinched so to speak.

OK