A used car website would be your best bet. Car values vary by local area. I just calculated the value of my 1992 Pontiac Bonneville to estimate what the vehicle tax would be when I move to Alabama from Delaware in June. According to Kelly Blue Book.com my car is worth about $1700 in Wilmington, Delaware; it drops down to $1450 when I run the value for Huntsville, Alabama. Its good for me because my tax is a little lower. Plus I only paid $2300 for the thing 5 years ago and I'm just crossing the 100K mileage mark.
Also, you really can't tell exactly how much a used car is worth without evaluating the overall condition. That Grand AM could have been run into the ground or have been babied for the past 8 years. We can't tell from your question.
The difference with the engines is simply size. I don't know too much about post 90's Grand Am's so I really can't tell you how reliable they are. I'm primarily experienced with full size GM cars.
With any car with that mileage you're always running the risk of driving the car off the lot and having something go out on it. I bought my Bonneville with only 65,000 miles on it and 6 months later I had to replace the AC compressor. (Luckily its been reliable otherwise) You never know. Accessories like that are usually going to go first; rubber seals get old and give way, electronic parts wear out, etc
If I wasn't desperate for a car, I personally wouldn't buy it unless it was dirt cheap. Cheap meaning offered for no more than 50% of book value.
Good Luck!
P.S When you look it up make sure to get the correct value class. If its being offered by a dealer the value is going to be more; if its being sold by an individual you'll need to click private party value. If its a private party sale its going to be worth less money. Dealers mark up cars like crazy!