Upgrading my iMac from 12 GiB to 16 GiB of RAM makes things feel faster. Actually, it simply allows more breathing room for multiple applications to run concurrently, avoiding things like swapping to disk.
I utilize a bunch of virtual machines (VMs) running different flavors of Linux and, at certain times, Microsoft Windows 8. Once in awhile, I run a couple of Windows Server 2012 VMs for testing and this requires a lot more memory than Linux. The upgrade makes running VMs and native OS X applications easy and problem-free.
If you need to run a number of applications simultaneously, more RAM is good. Having more cores per processor is another good thing, especially when your applications take advantage of them. Finally, a fast graphics card with a good amount of memory (say, 1 GiB), makes your games run more smoothly.
Don’t hesitate to upgrade your Mac or PC.