When it comes to buying a ball, how important is the rg and the differential in selecting one? By the way, what does rg and differential mean?

I think the best way to describe them so that you can actually know what they do.  
RG is length.  Higher the rg the long the ball will go.  
Diff is hook potential.  The higher the diff the higher hook potential.

When I'm looking at bowling balls those are the second and third most important thing i look at.  The first thing I look at is coverstock. Hope that helps.
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These are good things to know.  However, if you don't know much about bowling balls, it would be best to talk to your pro shop and have them help you choose what ball to buy based on what you want it to do.