Unless your budget is essentially infinite, you have to make tradeoffs, and if LR performance is your primary concern, it's easy to get sidetracked and spend money in the wrong place.
The two most important things are enough RAM and an SSD, both very cheap. I have 16GB of RAM and a 500GB SSD. CPU speed and number of cores would come next, but if you try for the state-of-the-art you'll waste tons of money.
You'll want a hard drive for actual photos (SSD is for apps and LR catalog, including previews). If you have an SSD, I doubt that the speed of the hard drive matters, but a fast one is very little more than a slow one, so get a fast one.
I don't think the GPU matters at all.
Don't forget your backup scheme. I use a USB3 drive permanently connected, drives that I connect only for backup, which I rotate, and CrashPlan cloud storage. It takes a long time to get the images uploaded to CrashPlan, but it's always running, and the images eventually get there. I also archive images to Amazon S3 Glacier from time-to-time, so I have 4 levels of backup.