Sorry you're right, I mis-read the manual (PDF's a great until the things you're looking at get split by page breaks).
That said unless you're using the onboard video at the same time as your 5770 (which works very well for an extra monitor or two), then I wouldn't worry too much about that setting. Not really sure what or how it would affect things as you would have to enable the onboard to test it.
The only thing I can see it being for is if you have more than 4 GB of ram and running 32 bit Windows, and that setting will force the onboard video to take the ram from above 4 GB and freeing up what it takes leaving a full 4 GB for the OS.