Do I have enough time to make a connection?

Immigration should only be about 10 minutes, customs will be at your destination. I think 2 hours and 40 minutes is plenty of time.

So as long as I don't leave the airport I don't have to go through customs on my original International flight from the US?
I'll just go through once I get to Spain?

Notice the operative word in Sam's advice is SHOULD. There are some assumptions - the plane is on-time, you get off the plane quickly, weather is good, etc. Remember you go through both immigration and customs. Two completely different activities. Customs is a walk through the Green door - nothing to declare. Immigration is where you passport, etc., gets checked. I would like a back up plan. Are you flying on one ticket? If you miss, then they will take care of us. A bit inconvenient but you will get to Spain.

Correct. I changed there on the way to Venice a couple of years ago.

This is an easy, much less busy airport than the major hubs. Barring any major issue, this should be a breeze for you with plenty of time to eat as well, if need be.

We are not flying on one ticket. We have roundtrip tickets to Stockholm. Then we decided to start our vacation in Spain instead and are now purchasing our tickets there. I have no back up plan :(

Is this a connection on one ticket or are you contemplating flying on two separate tickets. You did not mention the airlines or destinations.

On one ticket, you should be fine. Make sure to check the terminals that your flights use. Most international flights use the main terminal which is Terminal 5, but a handful of airlines (BA is one) use Terminal 2, and domestic flights are generally out of Terminal 4.

If you are buying a separate ticket, suggest you spend a night in Stockholm. There is no guarantee your flight comes in on time. If you miss the flight to Spain, you'll be vacationing in Sweden until you can book a new ticket at last minute rates, besides loosing the value of the original ticket to Spain.

I missed the post where you said you were flying U.S. to ARN to Spain on separate tickets.

1) You will go through immigration in Stockholm
2) If you have checked baggage, you will need to pick it up at baggage claim, go through customs, and then recheck and go back through security
3) Depending on the airlines involved, you may need to change terminals
4) If you don't have a checked bag, will you have a boarding pass for your second flight?

Keep in mind that if your first flight is delayed and you miss your second flight, you may have to buy new tickets.

Normally 2:40 would be fine, but you have no control over airline delays so it really depends on how much risk you want to take.

Since you're going on separate tickets you for sure want to spend the night in Stockholm to be safe.

We don't have checked bags.
I can pay 12.50E per person for missed flight insurance and fly out on the same flight the next day. :) It will just throw off my arrival in Barcelona.
I'm looking to spend around $150 if I stay the night in Stockholm and fly out the next day. Plus 1 day loss in Barcelona.
What can we do in March from 2pm - 2pm the next day in Stockholm? (besides sleep :) )