As the error says, you've got 2 options
Use Kies to install the upgrade.
Wait until your carrier's wind-up toy servers get un-overloaded.
A third option would be to obtain an image of the appropriate firmware (e.g. from samfirmware) and flash it via ODIN, though this is slightly more involved.
As an addendum, I was getting this myself with my own SG3GS3 on Virgin mobile Canada (aka Bell Canada) and I ended up waiting just about 2 days, from the initial reports of people upgrading on Sunday until I got it late Tuesday evening.