As Microsoft are desperate to plug the security holes in their product, they have issued service packs in the past for XP, & I believe Vista that have been available free with certain computer magazines. I`d have a browse in W.H. Smuts first. Also Google "Vista Service Packs" & read users reviews/experiences. It sounds as if you`re on dial-up.
No, it is about an hour, that's installation time not just download time. You have to leave your comp on and it stops and starts itself three or four times. I did it a couple of weeks ago. Can't say I've noticed anything different, but it must be important for security.
I have not noticed any magazines offering Vista SP1 on their cover CDs or DVDs.
I think this is that for various hardware reasons Vista SP1 is not offered to every Vista user, so by only offering it via Windows Update they can control who installs it.
I think the main reason for installing SP1 is that all future fixes for Vista will probably require you to have SP1 on.
I have read a few reviews of SP1 and some have noticed performance improvements but I think it is mostly security fixes (and a few other things under-the-cover that dont affect most users).
I would bite the bullet and install it, just set it running and go and do something else for an hour or so while it downloads and installs (go back to your PC every so often as it may be sitting there asking you to press an OK button or something)