I wonder how big are the perfomance hit on HD texture? I don't want to waste my 3GB bandwidth since I have a slow connection.

You can run it with your system, but there are some glitches within the pack. See the thread about texture glitches, which has nothing to do with the video card.

I'm not sure that all of the reported "glitches" are within the texture pack; pushing a computer beyond it's reasonable limits is bound to result in problems.