If you're semi-serious about gaming or just insist on having a browser or two along with some media player running in the background while you are gaming.
Or you work with a demanding program like Photoshop.
If not 8 GB is also reasonable, imo even 4GB is also reasonable if you have the OS and most programs on an SSD.
Most who read this forum would probably go with at least 16 GB of RAM I think.
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I would never recommend less than 8GB of ram in this day and age, unless you are a very light user who only uses a handful of browser tabs and don't often have other programs open.
My work desktop has 4GB of RAM and I silently curse to it every day. But I like to have a ton of things open, it's my way of working. Excel spreadsheets, PDFs, Shoretel, LogMeIn, quite a few Chrome tabs, Google Sheets in Chrome, and Outlook. The cursed thing often crawls with all that.
I can have all of that, a VM with my work stuff connected to a VPN, and like 50+ tabs across two Chrome windows, and my desktop with an i7-2600K and 16GB of RAM just keeps on cruising for days on end. I only restart for Windows updates.
If I am to play a heavy game like BF3 or 4, I'll usually restart to clear up memory and then enable Eyefinity. But I wont even do that for lighter games. I'll keep all my junk open and play Kerbal Space Program just fine on one monitor.
I may be more of an extreme user, especially when it comes to tabs. I hoard open tabs like a bad nightmare for some people. :awe: