Nike33, if you want to estimate things like this, Stirling's approximation to n! can be useful. It is:
n! ~ sqrt(2 * pi) * n<sup>n+0.5</sup> / e<sup>n</sup>.
Using this result, 600! / (300! x 300! x 2<sup>600</sup>) is approximately 1 / sqrt(300 * pi)
Furthermore, using the method of 1995 4u HSC, question 7,
1 / sqrt(300.5 * pi) < 600! / (300! x 300! x 2<sup>600</sup>) < 1 / sqrt(300 * pi)