This is how I updated my iPad (original 32Gb wifi 3G) to iOS 5 with an iMac 24" early 2008.
It took quite a long time.
First I backed up my Mac to an external drive.
Then I updated iTunes to 10.5. I did the Lion updates to 10.7.2 at the same time, just to keep things tidy.
Then I connected the iPad to the iMac.
Then I launched iTunes and started the update. Depending on your broadband connection, just the download could take an hour or more. If you're on cable or fibre optic, lucky you.
Next, the iPad backs up to your Mac. Depending on how much you have in the way of music, videos, apps and pictures on your iPad, it could take 15 minutes or more.
Then, the iPad verifies the backup with Apple. This is where things seem to have been failing for a lot of people: Apple's servers are being hammered by everyone from night owls in the UK to bright-eyed and bushy-tailed Californians.
If it fails here, you have to backup again. Every time it fails with an "Internal error", you have to back up again and verify the restore.
Patience is a virtue: eventually you'll get the connection. A progress bar will show on your iPad and the software will extract and update. Then the firmware will update. Then you'll be able to set up the iPad for wifi, location services etc. It'll even ask if you want to set it up for iCloud. (I'd hang on.)
That took another 20 minutes or so.
But don't disconnect the iPad. Because it'll still be copying over your apps. And then your music. And then it'll resync your photos.
From successful verification of the iPad backup to a fully restored iPad running iOS5 took the best part of an hour and a half.
Now, what does this puppy do?