If you are only restoring the app and not the data (Angry Birds but not your scores) the market will send them back down your way again automatically. That might be the easiest way to get them back.
If you apply a rom or theme - backup first!
If you don't know how to backup...Stop until you do.
hint: I name my backups like this: sg6.0_20100428 or ss4.2_20100626 to easily find them later. rom and date.
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