Diver- Your problem has nothing to do with 1080P and 720P resolutions. The only difference is where the upscaling gets done - either in the player, or in the TV. Expecting a big difference in an upscaling DVD player means that you are assuming that the scaler in your TV is junk. It's going to be a pretty subtle difference either way, unless your TV is crap.
Your problem, is that your new player just isn't as good as your old one. There's a lot more to video than resolution alone. Maybe it's defective, maybe it's just crap. It sounds like you need to find a better player, regardless.
Why did you drive 250 miles for a mid-fi DVD player? Online ordering is a few clicks away. The Oppo is well reviewed, supposed to be a good picture, but the build quality is low.
If you can scrape up a little bit of dough, the best DVD player I've found is the Onkyo SP1000 http://www.hpdirectav.com/item.aspx?eid=1&pid=92982 , it also replaced my Jolida CD player because it just sounds better, and does SACD and DVD-A too. Transport is the best I've seen on a DVD player, and video is near flawless.
Not necessarily plugging that product, the choice is yours... but in your case, I think it's time to return the new Denon and start looking into different players.