It depends on your focusing distance of your subject and the focal length of the lens you're using. And by "good DOF" I assume you mean a shallow depth of field, right?

Obviously you get a more narrow DOF with a faster lens at shorter focusing distance. And f2.0 is not a very fast for a 50mm. So, I'd say there is not much difference between f2.0 and f1.8 for your average shot focusing at, say, 10 feet away or so. So it kind of all depends on what you're doing and how isolated you want your subject to be. Your mileage may vary.

I have a Pentax f1.2, 50mm I use on my adapter. And I can say the difference between f1.2 and f2.0 would mean objects fading out of the field of focus are really blurry and hardly recognizable (f1.2) to objects that just out of focus and recognizable (f2.0).

You can always look at your 35mm lens DOF scale right there on the lens to get an idea of the difference between f2.0 and f1.8 for a given focusing distance if your lens has that range.