The best way to do the upgrade to iOS 5.1.1 (which is as high as your iPad1 can go) is to follow the steps outlined in this tutorial:
http://www.ipadforums.net/jailbreak...lsafe-method-restore-current-version-ios.html
Because the iPad is on iOS 4.x, I recommend this process as the older firmware made it difficult to simply click the Restore button in iTunes. This process walks you through the steps of (1) getting the iOS 5.1.1 firmware, (2) putting your iPad into DFU mode and (3) restoring to that firmware).
Oh, but do a backup just prior to ensure you don't lose your stock data and settings...
I would not recommend making/using a backup of all your jail broken tweaks. As you are making a major jump in firmware, I believe it will be best if you re-install your jail break tweaks one-by-one after jail breaking. Of course, this assumes you are intending to jail break it after the update.
Hopefully, this guide will get you to iOS 5.1.1. Holler back if you need more clarification. Good luck.
Marilyn