As of now, I only have the full set of size 128 textures, but I was wondering, what is the difference between 128x and 512? (I think its 512) Is it just the size?

Also, I do not understand what bumpmap textures are at all.
Anyone care to shed some light on the subject?

the thing is, 512 is BIGGER then 128.. so its easyer to work on a big sized picture then on a little one, (try to draw on a 50x50 picture and then try to draw on a 500x500 picture) its just alot easier, which provides better quality on the textures..
so because 512 is bigger there are more detailed.

In essence, it's a topic on pixels. A 128 x 128 resolution will give you quite a crisp picture, but you cannot really make it much detailed obviously because of the fact that there are less pixels in the image itself. A 512 x 512 resolution allows you to draw highly detailed textures as a result of more pixels in the image, giving you a more "High Definition" Image/Texture.

Hope that helped,
~Saddy
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I don't really know if that was an intentional pun, but bumpmapping is a way in which textures give off a more realistic effect with (seeming) bumps and elevation etc. In fact it's just a matter of light and how it reacts to a texture is all. Quite a complex topic in theory. I can't really delve into the topic further as I don't really know much more.
Hope that helped,
~Saddy

For the bump map texturing: http://forum.toribash.com/showthread.php?t=189220 There you go.