The question is whether disk performance is critical in your normal workflow, in which case you'd want to place the footage on the SSD.
I have a similiar setup with Sony Vegas and I observed that, disk performance is rather uncritical during rendering, because it's always the processor that's the bottle neck. But your situation may differ from that. Especially, if you have a very fast CPU and a very slow HDD.
The following configuration works well for me:
- OS on SSD to have fast boot time
- Vegas/Premiere on SSD to have fast startup time
- Video footage on HDD to have all footage in a single location.
- External drive for backuping your footage
- All other stuff: where-ever there is room left. Just make sure there is enough storage for your videos on HDD. 750GB fill up very quickly with HD footage ;-)
Disadvantage: May be slow with fast CPU's and slow drives. Project opening times may be slow as well.
Advantage: Fast boot and programm start. All footage in one place.
Of course, if OS+Premiere+Video fits to the SSD, put it there. This will give you the fastest experience. Plus the option to have a very fast backup to the HDD and a second slower backup to the external drive.
Advantage: Fast. Double backup.
Disadvantage: Not much storage for footage.
Put OS+Premiere to SSD and video to HDD. Place the one project that you're currently working on to SSD. This configuration is also very fast, but I sometimes experience issues with moving projects across the drives, so I try to avoid that.
Disadvantage: Possibly issues with moving footage/projects around.