drilling and putting a wall plug into spackle, taped up dry wall
Only the lightest of curtains should be hung with hollow-wall anchors in the first place, even if you use togglers (anchors with mechanically-deployed wings). Screw-in or expanding drive-in anchors are almost guaranteed to fail after a short time, and heavy or frequently manipulated curtains should be mounted into framing. 

To your question, if you fill the hole well with setting-type joint compound, which is much harder and stronger than "spackle" (a rather vague term leaving much room for confusion), and then you carefully drill for a toggle anchor, you'll probably be ok.