Thank you all for your help so far. I think there's a good number of people who are looking for something like this, yet there's not much info on it, and most of the scripts that do this already, cost money. In my case, I don't need the script, just the small piece of code to grab the banner with weight.
Technically it does work. However, I don't see how this would work with more than 100 banners. Maybe I'm just tired or (retarted)

But how can you do it with more banners when you're working with 1-100? Or, when you're setting up the weight value for the banner, how do you not duplicate?
Is this the same as doing percentage values? Or is there a way to do this with just percentage based?
superdezign You make good points. And just for the record, this is not for people paying for banner spots.
Do you have any suggestions or code samples to do that? Randomizing higher values? But what about lower values?