Its all a balancing act. If your server is at 90% diskspace, then you're going to need to upgrade to get more diskspace or delete stuff to make room.
If your site is starting to throw errors in your activity log about time taken to process stuff, you might need more CPU power, like many people uploading tons of audio. Even if you have space to fit it on the server, the conversion process continuously running while also having many visitors to your site may mean you need more CPU power.
If you had a Ton of content, but not very active, diskspace is your main issue. If you had a ton of visitors to your site, CPU. If your visitors main activity is streaming video, CPU, Memory and Bandwidth.
CPU Cores
Quote: Server CPU (Processor) Cores
This is the number of processors in the server
The higher the number of processors, the larger load the server can handle.
Memory
Quote: Server Memory (RAM)
The amount of memory (in Gigabytes) that the server has
The higher the amount of memory, the more simultaneuous users can be on your site at one time.
SSD Disk Space
Quote: Server SSD Disk Space
This is the amount of Disk Space (in Gigabytes) on the server.
All Jamroom Hosted servers use premium enterprise grade SSD (Solid State Disks) - these can be up to 100 times faster than older hard drives.
Network Bandwidth
Quote: Server Network Bandwidth
This is the amount of dedicated bandwidth (in megabits or gigabits per second) available to the server.
The higher the bandwidth the more simultaneous users can be on your site at the same time. If your site streams media (audio, video) higher bandwidth is better.