There are TONS of posts about Yellowstone and the routes there. You have many possibilities, depending on how much time you plan to take for the trip (I strongly recommend a minimum of 3 weeks total from the East coast), and what your interests are. South Dakota along I-90 has a lot of great tourist spots, especially in the Black Hills west of Rapid City (Mt Rushmore etc etc), and Nebraska along I-80 and US 26 has many interesting pioneer sites. There's also Devils Tower in n.e. Wyoming and Little Bighorn Battlefield along I-90 in s.e. Montana.
Try a search on this site for Yellowstone, especially under the Campgrounds & Attractions and the Roads & Routes forums, and you'll find a wealth of information. Good for you for planning ahead too; Yellowstone is a popular place and reservations 6-12 months ahead are a very good idea.
We have been from NJ to Yellowstone several times. Have taken both I80 across PA to Ohio Turnpike, etc. and the Pennsy Turnpike to connect with I70. Personally, I prefer the southern route as it avoids the Ohio, Indiana and Illinois toll roads and the traffic mess in the Chicago area. With the I70 route, we pick up I74 past Indianapolis to I80 near Davenport then I29 near Omaha up to I90 to go across South Dakota. There isn't much to see until you get to South Dakota unless you really enjoy seeing corn fields.
Agree with kknowlton that 3 weeks is a minimum for this trip distance to be worthwhile (unless you have several drivers and really enjoy driving).
Several days in Grand Teton NP just a few miles south of Yellowstone would be time well spent.
Good points made by CloudDriver on the routes. The "mess around Chicago" is just that - AROUND Chicago. I-80 itself isn't bad now, the major construction having been finished, and if you want to head farther north, say to I-90 or I-94, you can continue on 80 west past Chicago, then head north on I-39 near Rockford, IL, or on I-35 or I-29 in Iowa, to connect to I-90. I-70 to I-74 to I-80 is also a good choice if you're coming from PA Tpke country.
Well it really sounds like the southern route using the Pa trpk will be the route and going through S. Dakota to see all it has to offer as well. Thanks to all for your help
Offline
