You would need to know more than just "X = 5". We would need to know something about the distributions and the moments of those distributions of the random variables of interest. If for example you wanted X and Y to be bivariate normally distributed and you know what you want the marginal distributions of X and Y to be along then we know that Y conditioned on X has a normal distribution with parameters that depend on the correlation and the means and variances (and of course on the observed value of X).

The wikipedia page gives the exact distribution of interest in this case: http://en.wikipedia.org/wiki/Multiva..._distributions