You really have to test the specific type of thing you want to render and see how it affects your system.
First watch the CPU - is it maxed out? If so, you'd likely benefit from a faster CPU.
If not, why? Take a look at the memory usage - if it's maxed out, then your CPU is likely sitting around waiting for frames to be swapped in and out of RAM. Adding more RAM will likely help.
If neither the memory nor the CPU are maxed out, look at your storage system - is it reading data at or near it's sustained transfer limit? If so, adding a faster storage system will allow data to load into memory faster, which will allow the CPU to do more work.
If your memory isn't full, your drives aren't running at max speed, and your CPU is still not maxed out it probably means your software isn't optimized to make use of all the cores in your processor. You'll either need better software, or a CPU with a higher per-core clock speed, to speed things up. It's also possible you don't have your software configured correctly to make use of the available resources - for instance After Effects often needs some options configured properly to really max out the CPU when rendering.
The problem is, whatever the bottleneck - once you eliminate it you'll likely just bump into the next one.