Should help to have an ext 7200rpm drive with all your media and project files on it, then leave the internals alone. You'd get better performance from upgrading them, but not sure how much. You don't want that stuff on your system drive, anyway.
Media and large project databases benefit the most from faster drives. The OS and an app running won't eat up that much bandwidth. You would see the OS and app be a bit snappier, but you won't see a HUGE performance upgrading the internal OS drive. Not as much as you'd see getting fast media drives.
I had one of those. I do miss it for the size as far as a little editing machine.
I upgraded the two internal HDDs to SSDs. It made the machine boot up way fast, apps came up quickly, all the benefits of an SSD. But, I never was able to store a lot of files on the internals of the machine. So, if your okay keeping those 1TB drives I would say do that.
Just my .02.