If you want something inexpensive, the best solution is probably the Tricaster by NewTek:
http://www.newtek.com/tricaster/
If you say, "That's too much money", you then are faced with assembling components on your own. You would need a lot of video engineering knowledge. And if you are asking these questions in this forum, you don't have that knowledge.
Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA