I don't get a lot of you guys who are having trouble downloading 10.5.7 using the Software Update from the Apple menu.
If you post here, it means (presumably) that you read this forum regularly. If so, you would have read infinitum that, whenever there's a major OSX update, the safest thing for your system is to download the Combo .dmg file -- the one that contains all the updates from 10.5.1 through to the latest -- and re-installs everything nicely.
Ever since about 10.5.2, I've been doing that without trouble. Surely by now this is widespread knowledge, at least for people who regularly read this forum. And if you're posting here, chances are you should be regular readers.
Sheeesh, you guys.