I would probably go for 3 nights in Yellowstone and 1 in Jackson if you can swing it.
Peachbelle is right that some do the Upper Loop one day and Lower Loop another day. In my mind, there are 2 major areas on the Lower Loop, the Upper/Midway/Lower Geyser basins and the Canyon to Fishing Bridge area that can easily have 1 day each.
Your Jackson to Pendleton day is going to be pretty long. The first couple of hours will be on 2-lane road, then you'll be on Interstate the rest of the day.
I would be happy to do itinerary suggestions. You basically have 1 full day and a part of the next day because that is your day you are going to Jackson.
Day 1: You will arrive late in the afternoon from Spokane. I would exit the Interstate at Cardwell. Once you turn right off the exit ramp, there is a big brown sign that routes you to the left to *Yellowstone*. I go straight onto a smaller road and it will connect to US 287. This is a few minutes shorter. You will follow 287 on in to West Yellowstone. If you get there early enough you may want to go ahead and buy your entry ticket at the Yellowstone Visitor Center in West Yellowstone. It is next to the IMAX theater. There is also the Grizzly and Wolf Discovery center if you have kids and they are interested. You can also drive in to the park and look for animals. It is likely you will see bald eagle, trumpeter swan, elk and bison between West Yellowstone and Madison. If you have bought your entry ticket, you can go to the *Prepaid Pass* lane at the entrance gate which is usually quicker.
Day 2: I would make this your geyser day. Turn right at Madison Jct, after you cross the Gibbon River Bridge, turn right in to Firehole Canyon. When you get back to the main road, head in to Old Faithful and the Upper Geyser Basin. Go to the Visitor Center first, write down the eruption times for the predictable geysers on the brochure available at the desk (50cents). Try to work some of these in on your walk thru the geyser basin. Save some room for ice cream at the Old Faithful Inn. You get it on the 1st floor and can take it up to the 2nd floor deck overlooking the geyser basin. In August, the gesyer basin can be quite hot. You will need sunscreen, hats and sunglasses. After you finish this area, head back toward West Yellowstone stopping at the geyser basins you went by this morning...Black Sand, Biscuit, Midway and Fountain Paint Pots. Also drive the Firehole Lake Road. Overnight in West Yellowstone.
Day 2: Head in to Canyon. Make a pit stop at the Visitor Center, then do the North Rim. If you only do one viewpoint, Brink of the Upper Falls is the first one you come to. Go to the South rim, at least do Artist Point which is the classic view of the Lower Falls. From this area, go south thru Hayden Valley, looking for wildlife. Stop if you see anyone out with long lenses on their cameras! Do Mud Volcano. At Fishing Bridge, stop, park and walk across. This is often a good place to see wildlife, believe it or not. From here head south around the lake. If you are not geysered out, do West Thumb geyser basin, then out the South entrance.
If you can possibly add a day, you could do the Upper Loop on day 2 and then do this day 2 on your day 3.
Edited: 9:10 pm, July 12, 2012