Line capacities is only used for the scale of the statistics graphs, you should set it to whatever your internet connection is capable in download / upload.
The bandwidth limits set the maximum for amule to upload or download. You should set your download limit quite high as it will rarely be maxed out (near to the maximum of your lines download speed). Upload is a bit different, you should leave some room here so that you can still surf the web while amule is running. If you have a possible upload (of your line) of 16 kB/s /thats kilobyte, which means 128 kilobit) you should set the upload limit to 10kB/s...
Slot allocation is used for uploading and decides how much bandwidth every upload gets. If you set your upload limit to 20kB/s and set a slot allocation of 4kB/s there will (normally) be 5 uploads of 4kB/s each.
I hope this made things a little clearer, if there are questions left feel free to ask.
