For older computers: Older computers that came with at least Vista, are sometimes limited on how much RAM can be installed even if your running a 64bit system (for example, this PC that I am on... Is from 2007 and came with Vista, I have maxed out the RAM to 4GB which is the limit of the motherboard) so what do I do if I want it to go faster?
The best upgrade for an older PC is installing an SSD. In most cases, a mechanical Hard Drive is the bottleneck in old or new systems.
For New Computers: You should have at least 8GB RAM (16GB RAM is the norm), An SSD for the system drive, and mechanical drive(s) as data storage.