In my house I use a screen for one setup and a painted wall for the other and there is definitely a difference.
The screens (either a Stewart GrayHawk or SMX Acoustically transparent - both 110") produce a more vibrant, brighter, and uniform picture. Blacks and contrast are also better with the screens.
So if the screen is so much better, why do I use a painted wall? Because it was cheap and easy and it is for our bedroom setup. Given that we only watch it right before bed, we aren't that picky and it gets the job done.
If you want to get everything you can out of the new setup, spend the money and get a screen. BTW, if you want to really make that screen pop, try painting the area around your screen flat black. This will soak up stray light and reflections and make a noticeable difference.
George