craftgrrl | New Sewing Machine Foot?

I discovered a $10 gift certificate to Sears buried in my stuff. Seeing as I have a Kenmore from the '60's, I want to try getting a new foot as a treat. I know the parts are compatible.

I do a lot of garment sewing and would like to emulate a serger. I already have a zipper foot and a rolling foot and a buttonhole attachment. I was looking into maybe something to help me with knits too. Sears has piping feet, cording feet, applique feet, and hemming feet, to name a few.

What kind would you get?? (not necessarily one of the ones I listed, because there could be other kinds that they carry, like a walking foot.)