gains from vectorization

I am working with a g95 compiler and I need to increase the efficiency
of my code. one of the things that I came across while browsing the
archives of this group is vectorization. However, after enabling that
option using the flag -ftree-vectorize, I don't seem to be making any
significant gains in performance. Can anyone suggest something? Is it
also something that's processor dependent? If that's the case how would
I know if my processor can do that pipelining operation? uname -a on my
system gives the following output:

Linux s5 2.6.5-7.151-smp #1 SMP Fri Mar 18 11:31:21 UTC 2005 i686 i686
i386 GNU/Linux

.