$2000 Cluster purely for computations

I've been doing extensive research for my first build that's going to be a 3-node cluster purely for computational chemistry use. I am looking for any advice and suggestions to help me along the way. I'm not expecting to begin building the system at least until mid-September.

The system will run a version on Linux and the Gaussian 09 computational chemistry package. I plan to use it for intense and very long calculations, so speed is the biggest concern. If anyone has any knowledge on this software and can point me in the right direction, it would be greatly appreciated. This will be an ongoing build for the next few years as I attempt to create this in home system from the ground up instead of buying one.