Yes you "should". But you should not feel obligated to spend as much as if you were going to the wedding. I usually do gift cards or checks with a nice wedding card.
When I go to a wedding, I usually spend a lot more since they are spending a good chunk of money on me (and wife) for food, drinks, etc.
If it is someone you don't like and question why they sent you an invite in the first place, go ahead and don't send them a gift so you can cut them off completely.
Edit: Best wedding etiquette is to RSVP. Non RSVP'ers piss me off. Sucks having to call everyone cause they can't take 30 seconds to RSVP online.
