OK, think about your travels through Org/IF/SW.

If you stutter badly for a while, then things eventually settle down and start being playable, then your primary problem is the time it takes to get textures from disk into RAM. That is a function of disk I/O, thus an SSD would benefit you the most.

If your FPS varies wildly the whole time you're there (one second you're seeing 30+fps, then stutter/stutter/disk grinding/stutter, then 30fps again) that usually indicates not enough RAM as things constantly swap in and out of RAM to your pagefile on disk.

If your FPS is very low all the time and your disk does not sit there grinding constantly, that's either not enough CPU horsepower or not enough GPU horsepower.