I too am looking for a way to limit downloads...
The reason just pausing them all and unpausing one by one is a pain - who wants to sit by the computer all day, double checking every few minutes when one download is finished and it's time to start another one. Sure, might be fine if you only have a few things in que, but I'm trying to get about 40 language book chapters, I don't want to babysit all day!
iTunes seems to set the limit at 3 simultaneous downloads, changing it to one would be a blessing for some of us.
Dell Windows XP
Dell Windows XP