I'm not sure.. Unfortunately most Acid Pro 7 videos were made in the youtube 1.0 era, and are somewhat difficult to come across now shows the mixer functionality, and there are other parts available from that channel in a series. Then there's vids from SCS's youtube

robertNC, thanks! I guess I was looking for some first hand knowledge from people who have made the transition from 3 to 7. I figured I'd upgrade now before the current company messes with it too much. Thanks again!

The Sonic Foundry/Sony team from Wisconsin that made Acid Pro 7 are likely handling most development duties of Acid Pro 8, so I don't think you have anything to worry about in case of an eventual upgrade - they probably will have been with Acid through its whole existence

Cool! I really like the old Sonic Foundry software and hope you are right. I'm sure there's a lot of new features going from 3 to 7. I wonder what 8 64 bit will bring? Thanks again. David

Some of the major features added between Acid Pro 4.0 (What I started on) and Acid Pro 7 is -
Multi Track recording. This is HUGE if you're recording a band or more than 1 channel at once. Your mixing board has to have the ability to multitrack, but now Acid can record as many channels at once as you want.
I think the -/+ buttons are a new thing since Acid 4.. Now you press - to shift your sample down one semi-tone or + to shift it up one semi-tone, up to 24 semi-tones (2 octaves).
I don't know if Acid 3 could run VSTs, but that's huge.
The "U" button now reverses a sample so it plays backwards.
The rendering engine sounded better between Acid Pro 6 --> 7, but I haven't used 6 in a while so I could be remembering that wrong.

The biggest thing for me though was the multi-track recording. That's essential for recording drums or multiple vocals at once or even putting multiple mics on one instrument.