Get 2x16 GB RAM, or 2x8GB RAM at the very least. Photoshop eats memory for breakfast once you get into serious projects.
For the rest, is this a professional build, a semi-pro build, or just something to learn Photoshop with?
UPDATE: To put the RAM issue into perspective, Photoshop doesn't have an option to set the maximum RAM it's allowed to use. It has an option to say "Hey, leave this much for the OS!".