Assuming a 2.6m viewing distance, you can see the full resolution of 480i with a 25" set. So yes, I believe that you can tell the difference between 480 lines and HD resolutions on a 40" set.
You may be able to "perceive" a difference between 720p and 1080p on a 40" set (through vernier acuity). But you are unlikely to be able to actually resolve details at the full resolution of 1080p on your 40" set unless your vision is much better than 20/20.
With 20/20 vision, you can identify a letter (such as "E") only with a subtended angle of 5 arc minutes or more.