Quick Question for the Gurus (about Zoom)

[fowlmood2]

OK guys, to start off with, I am a novice with a decent camera. I like to take pictures but do not have any plans to take a lot or make large investments in this hobby. However, I would like the capability to get "closer" on some occasions and I am looking to improve my zoom capability a bit and want your opinion. I currently have a Sony Alpha (DSLRA-100) which came with a AF18-70mm lens. I also have a AF70-210mm lens. I have three different options that I am considering:

1,) Purchase a 2X teleconverter and use with my existing lenses. With the teleconverter, the autofocus feature will work (assuming enough light) and I will be able to get to 420mm with my current 70-210 lense.
2.) Purchase a larger telephoto lens. The ones that I am willing to pay for all have only manual focus but would allow more zoom (500-100mm).
3.) The last option is to get a AF75-300 lens which would esentially render my AF70-210 useless.

Thoughts? What are the pros/cons of each of the options?

Thanks in advance for your help.

[GordonGekko]

Generally speaking a teleconverter on a zoom lens doesn't give great results...particularly a 2x TC....