Do I have to see Fantastic Four to understand Rise of the Silver Surfer?

To understand? No, not necessarily. If you just know who the enemy is, that's all you would really need to know. And everyone pretty much knows who their enemy is. Whether you're a fan or not :P

oh its better if you just grab it..

as long as you remember the following equations you'll be fine:
Dr. Doom = bad
Fantastic Four = Good

imo the first movie was pretty bad, and the second is a little better but still not very good. If you still want to watch it, I don't think you'll have a problem.

Prob. A good idea if you catch the first one... Character development and such..
besides #1 was way! better than #2 IMO. :P

it's better to watch the first f4 movie in order to understand the plot of the second one