How to evaluate Bessel functions $K_{i x+\frac{9}{4}}(2\pi)+K_{i x-\frac{9}{4}}(2\pi)$ with $x>1.5*10^8$ in Mathematica 7?

Question

I am trying to evaluate Bessel functions $K_{i x+\frac{9}{4}}(2\pi)+K_{i x-\frac{9}{4}}(2\pi)$ with $x>1.5*10^8$ in Mathematica 7. This function is the first Polya approximation to Riemann $\xi(s)$ function.

In Mathematica language these two Bessel functions are represented as "BesselK[I x+9/4,2Pi]+BesselK[I x-9/4,2Pi]"

At $x=1.0*10^8$, I got "$-1.28351162994662*10^{-68218805}+0.*10^{-68218820} i$"

At $x=1.5*10^8$, I got error message: "General::ovfl: Overflow occurred in computation."

Thanks- Mike