Obviously, the first performance enhancement you do on any computer you own is max out the RAM. RAM has gotten cheap, and adding more of it to almost any computer will make it faster without requiring any other modification (or any great skill). The next thing you need to do, says Larry O'Connor, the founder and CEO of Other World Computing (OWC), is move from a "platter" hard drive to a Solid State Drive (SSD). Larry's horse in this race is that his company sells SSDs, mostly for Macs. But he's a real evangelist about SSDs and computer mods in general, even if you buy them from NewEgg, Amazon or another vendor.
A big (vendor-neutral) thing Larry points out is that just because you have a Terabyte drive in your computer now doesn't mean you need a Terabyte SSD, which can easily cost $500. Rather, he says, all you need is a large enough SSD to contain your OS and software and whatever data you're working with at the moment, so you might be able to get by with a 120 GB SSD that costs well under $100. Clone your current main drive, stick in the new SSD, and if your need more storage, get another hard drive (or use your old one). Simple. Efficient. And a lot cheaper than buying a new computer, whether we're talking about home, business or even enterprise use. (Alternate video link.)