Where to install LR catalogs ?

Quick answer is C drive.

The Why is below.

I have about 50,000 images in the LightRoom folder on my C drive in My Pictures and it is just about 8GB. My C drive is a Samsung SSD of 256 GB but even if you go with a 128 you are only using a very, very small part of it. I do think it is better to have a 256 than 128 C drive at todays prices. This computer is Win 7 64 bit with i7-3770K and 16 GB of memory.

The images are on D drive which is two 2TB drives in Raid 1. I back D up to internal E and two external drives, one of which is always in a fireproof safe.

Hope this helps.