There are a lot of moving parts here, so you kind of have to pick apart the system one piece at a time to figure out where you're getting the quality loss.
Start with the connection to the camera - plus the composite connection directly in to a TV. What does the picture look like? I don't suspect your problem is here (especially in svideo vs rca, there is little difference in my opinion) One thing I do like about the D70 is it has better optics than most cameras - so you should have a relatively good picture. You might be able to tweak some things via the remote control (i.e. white balance, contrast, etc) but on the whole, you should have a relatively decent picture.
Next culprit down the line would be your capture card. First of all, check to see what its max supported resolution is, and where to change what resolution it's capturing. This might be in the Windows Control Panel or some other manufacturer supplied software/control. Many capture devices are set by default to only capture at 320x240. If your capturing at that resolution it doesn't matter how "high" you put the webcast software on, it will only just scale up the already bad picture you are capturing. Try to put your capture card into 640x480 mode. This will match the "high" setting on the webcast software.
Next, check the internet connection on the receiving end. I'm not sure if the church's streaming servers are 'adaptive' - but theoretically, a slower/crowded connection on the receiving end should cause the webcast to step down the stream to a lower resolution/bit rate. This would also affect the perceived quality at the receiving end.
