I would suggest keeping the catalog and preview files on the SSD. It's questionable whether you will notice any real performance improvement. Your images could be moved to another drive. There have been tests published here that indicate that having the images on the SSD drive doesn't provide any performance improvement.
I've got close to 5 TB of image files, so that wouldn't work! : )
As I'm working on an early 2009 Mac Pro
2.66 GHz Quad-Core Intel Xeon, 24 GB RAM, NVIDIA GeForce GT 120 512 MB, OSX 10.10.4
Lightroom and Photoshop CC 2015
what's the most cost effective way to speed up Lightroom?
The system is most definitely bogging down. Lots of spinning balls, slow writes to screen.
It's an old box, so I don't want to put too much into it (saving up for a new machine), but if not an SSD, video card? Other?
Have you tried turning off GPU acceleration to see if that helps?
Oh yes. First thing I did after LR 2015 CC 2015.
I know I need to speed up this box, it's old and apart from the 24GB of RAM, I haven't really done much too it. It's figuring out what's the best bang for the buck to speed up my primary pieces of software, Lightroom and Photoshop. Perceived wisdom says putting in an SSD, but would a video card with more memory be a better bet? Lightroom in particular can really lag in updating my screen.
You've got plenty of RAM, in my opinion. I'm using a Windows box, 8 GB RAM. And I have acceptable performance. I can't comment about another video card. I'm just using integrated graphics on the motherboard, no graphics card.
Maybe I'll post another question specifically about my machine. It's been surprising difficult to find out what my best course of action is (apart from dropping lots of money on a whole new machine!)