using a continuous-valued variable as fixed effect

Hi all,

I'm a newbie with linear mixed model.

I just would like to know if it is possible to use one or more variables 
assuming values in a continuous range (e.g. a frequency values in Hz) as 
fixed effects, in a linear mixed model, possibly together with (or 
without) categorical (binary) effects, using lmer4 package.

In case: how to do it and how to read the results, in terms of 
statistical significance?

Can anyone explain if in this case mixed models are actually different 
from a simple (multiple) linear regression?
Why should mixed models be preferred to linear regression in this case?

Thank you in advance.

Francesco