HD content -> HD display - no benefit
SD & DVD -> HD display - potentially huge benefit
When you go from an image that has 1 million pixels at full resulution and put it on a screen that has 4 million pixels (number of pixels randomly selected for example purposes) the processor has to invent/create an additional 3 million pixels to fill in all the extra space in a realistic way.
Imagine that one processor only gets 1 in 2 of the invented pixels 'right': not a pretty picture. At least not compared to a processor that gets them all right.
With processors that are not so good, you get scaling artifacts like jaged edges, loss of fine detail and, um, other stuff?? I don't remember all the details, but picture quality noticably sufferes.
These days, stock scalers are much better than even a few years ago. In my system, the scaling is handled by the graphics chips built in to my mother board and for DVD to 1080P does a good job: good enough for me anyway.
SD tv signals to 1080P presents more of a challenge so my scaler could possible not be good enough. Luckily I don't watch TV any more so its a non issue for me.
Before spending money on an outboard scaler, I would personally try to do a direct comparison between what you currently have and the outboard scaler.
