Long story short, I live in an L-shape apartment, my computer in one end, and the Internet source in another. The distance isn't great (6.3m by 3.6m), but there are a lot of walls in-between if you go in a straight line, I did a test, the speed would drop to 12% of its capacity.

Question 1: What is the difference between a repeater, an extender, or a booster?
Question 2: Does any of those exist in a form that I don't need to plug them into any power source?
Question 3: I read that I can reuse my old router as a repeater, except that I plan to use 802.11n in the future, and the old router is a 802.11g, that would end up dragging the speed down, doesn't it?
Question 4: My computer does not have a network card, is a Wifi USB dongle reliable enough to replace it?

Thanks for your time.