What's the best Memory card for Sony HX9V Camera ?

SD memory card speeds are rated as "class" or "X"
Here is a good explanation of both:

http://en.wikipedia.org/wiki/Secure_Digital#Speeds

SD memory cards are also rated for storage capacity:

SD = up-to and including 2 gig
SDHC (high capacity) = 4 gig through 32 gig
SDXC (extra high capacity = 64 gig and larger

The Sony HX9V camera and any camera that offers HD (high definition) video recording should be using a card that has a Write speed rating of Class 6 (or better) or 40x (or better).
Usually write and read speeds are pretty close together.
But write speed is used in the ratings.

A slower card usually works for HD video, but it will likely drop frames and/or drop information or both.

If the playback looks jumpy, it usually is an indication of dropping frames.
(instead of getting 30 frames per sec (fps), you might just capture about half that many frames)

If you have some slower SD cards, you can still use them for still photographs.
Just don't use them for video.