The basic system would include:
whatever source you have (cameras, computer, dvd player, etc)
and all those sources get sent to a video mixer. If your video mixer doesn't up/down convert (to whatever output you're trying to scale to), you'll need to do that first.
For example, if I only had svga inputs into my video mixer, I would have to upconvert my dvd player before I sent it to my video mixer.
If I was sending s-video or coaxial from my video mixer, I would have to downconvert & scale my computer's output before it got to the video mixer.
Once you have your mixer, you can use whatever video capture card you want to as long as it has the correct inputs. Then you can continue to use U-stream to upload.
As far as video mixer's go, do a quick google search and you'll find several in several different price ranges that will do what you're looking for without having to get a tricaster.
That's about the easiest way to do it without spending more money. And remember, it's WHAT you're streaming that matters most...poor editing/production won't be solved by better equipment alone
JF