Being the proud new owner of a 42" Panasonic and having struggled with the connectivity, here's what I found:
On the plasma you can switch between three inputs - PC (15 pin D-sub), Component or AV. AV can either be composite OR s-video not both.
I have my DVD connected direct to the component, with everything else connected via my AV amp to the composite AV (Using scart to video and l/r audio cables). In time I hope to get a SCART to D-sub cable to connect up the Sky box using this method to get a slight improvement in quality over the composite method, but honestly whilst you notice the difference in quality with the DVD (having tried both component and composite methods), the difference for Sky/VCR etc is very slight.
I tried connecting the s-video at the same time as the composite but found that the screen always defaulted to s-video even when no signal was present so I don't think your plan will work (unless I did something wrong when connecting up).
The advantage of this method is that you can use the AV amp as a switch hub (only one button to press to get sound and video), and only need to change the plasma input mode when watching DVD.
Hope this helps.
BTW I originally had the tuner box and card but sent it back - the picture degradation was terrible, so the £450+ has gone on cables instead!
Current stack:
Technomate Digital Satbox
Pace Skybox
Panasonic VCR
Pioneer DV-747 DVD
Yamaha RX-630
Mission speakers
Panasonic TH-42PHD5 (1024x768resolution)