I think
http://www.google.com has more than enough answers.
1. donot know what Flashget is, however a download accelerator is 'supposedly' able to speed up your download. However, for people unexperienced with the Internet, I suggest you leave ANYTHING that promises you that your PC will become faster
alone2. I have no idea, as I said, I donot trust them.
3. Depends on what software you are downloading. Again, google.com has your answer.
4. google.com has your answer.
5. That depends on the type of software. Nothing that promises to speed up your PC for free can be reliable. Unless it is recommended by professional users. You are most likely using Windows. Almost all the software out there is compatible with Windows.
6. You are appearantly new to the computers and/or the Internet - I suggest you do some research before asking questions or downloading files that may be dangerous.