I want a very fast linux.
 

WHat is the fastest linux thats useable for desktop use???\

Thanks.

That depends on how much work you're willing to do to make it fast. Just out of the box, I've been told Slackware and Gentoo are the fastest. If you do a base (text install) of Debian and then build your system on top of that, it is very responsive as well.

You'll also gain some speed by recompiling your kernel and cutting out anything you're not using.

Speed on the desktop will also depend on the desktop environment or window manager you choose. If you want it to be fast, don't use KDE or Gnome, use something like IceWM, windowmaker, or fluxbox.

And the thing that makes linux really fast is more RAM. :D

I would have to say Gentoo and Slackware are real good canidates for what you need but you are going to have to put in the effort and frustration to obtain speeds that you seek.

mainly frustration.
If you are new to linux, which I suppose you are. You might want to try a regular beginner-distribution, until you are more comfortable with the concept of linux.
After that you can make pretty much any distribution as fast as you want by using a light window manager. This is pretty easy to do. You should know some shell basics though.
If you have an old pc, or you really just want speed, try damnsmalllinux or feather linux and the like.