So, about the AGP 4x-8x, the theory is that 8x runs faster than 4x?

It should be faster in theory. If it's not implemented correctly on the
motherboard or video card it probably won't be though like in my case.

You also have to keep in mind that just because you have 8X does not mean it is
being used like with todays games. The bandwith is there but there is nothing
there to stress it.

Lets say you had an 8 lane highway and a 4 lane highway both going to the same
location. Would a single car get to that location faster using the 8 lane
highway or the 4 lane highway? See it wouldn't matter.

I will run some bench today at home with a program call Sandra something (a
friend told me was a good program to test hard), does anyone know it? it's
good enough?

For graphics testing you should try 3Dmark2001 or 3Dmark 2003. You could also
try running a few newer games and check out the framerate.

Sandra is good for general testing and for system information. I use it but not
for graphics testing.

About the aperture size, it's cool, I understand now, I will try 128mb

That's a good enough setting.