Telephoto lens for Pentax Kx

Hi,
A Belated Merry Chrismas to you and the DP Team.
I am planning to buy one of the two lenses for my Pentax Kx, the Sigma 70-300 F4-5.6 DG Macro or a Tamron AF 70-300mm F4-5.6 Di LD Macro. These I find to be auto focus and within the price range I would look at. Pls also let me know what do I need to order to be sure of getting the correct mount for my Pentax Kx. If this does not fit into my camera, returning it from India will be difficult.

What are your views on these lenses?
Pls advice if these have the correct mount for the Pentax Kx.
Also, to enhance the power, I am planning on buying the Vivitar series 1 high def 3.5X. Pls advice if Vivitar Series 1 High Definition 3.5X Telephoto Lens compatible with Tamron / Sigma 70-300mm for pentax kx.
I would also need a closer picture for the way this series 1 is mounted on the lens,
I would like to know your views on which one to use without losing the brightness and the crispness of the image and ofcourse without loosing too much cash.
I plan to use this lens for my visit to the wild life sanctuarys and taking picture of the Snow Peaks of the Himalayan Range on my next visit.

Looking forward to your reviews and accordingly will buy the lens.

Cheers

Samindra