If you don't know where to begin, I third the recommendation for the "Crucial Memory Advisor Tool" at http://www.crucial.com/
. I check there, then find an equivalent product from Newegg that's almost guaranteed to be cheaper, then check the price of that same product from Amazon.
Speccy is a free program that is also super handy if you want to know everything there is to know about the hardware of your computer. It's useful for identifying how many RAM slots you have and how many are populated, what speed your processor is running at, temps of various components, RAM timings, etc. Really neat little program.
For a nice computer (e.g. my enthusiast/budget hybrid for audio production and gaming at home), I build from Newegg/Amazon. For a cheap computer, desktop or laptop, I buy from Dell Outlet. Once a month, they have a sale for 20-25% off their already low prices. And if you don't juice up the specs and don't need a powerful CPU, their buying power is ridiculous.
The Dell Outlet search tools are laughably bad, so just pick Business if you want Windows 7 or Home if you want Windows 8, then click View All Laptops if you want a laptop, or View All Desktops if you want a desktop. Then do not
filter with the check boxes that don't work, just sort by price. Find the cheapest computer with a processor you approve of for your needs, then assess how much extra it is for the version with 8 GB of RAM. If it's a slight price change, buy the one with 8 GB of RAM. If it's a big price change, buy the one with 2 or 4 GB or RAM then purchase the extra RAM from Newegg right away. Do not
buy an SSD from Dell, overpriced crap. But buying something like the Samsung 840 EVO is a fantastic upgrade, and it's not that much of an extra hassle to install the OS fresh on a new SSD if the computer is brand new and doesn't have programs or files on it yet anyway. That's my secret for easy, cheap computers.
But be careful: they will skimp on components if they can. The most recent desktop I bought like this surprised me when it came with only 2 RAM slots and only 1 PCIe slot. But I'm sure I could have avoided that if I paid attention during ordering.
Make sure that when you upgrade your RAM (with any computer, ever), you install it dual-channel (or tri- or quad- if it supports it, most don't). This normally means tossing your old RAM and using even-numbered quantities of identical new RAM sticks. But it really makes a difference in speed. Really.