I have a 2010 MacBook Pro with 4 GB of RAM. This has served me well for years, but lately it just doesn't seem to be cutting it. This computer came with Mac OS X 10.6 and it now runs Mac OS X 10.9. I don't know how much memory 10.6 used, but now when I start up the computer, log in, and check how much memory I am using it is 2.6 GB before I start any programs to do work. I can see that many programs have started up automatically and these include programs like Dropbox, the Finder, Alfred, and Dashboard.
I did some poking around and found that web browsers now use a lot of memory. The Activity Monitor screen shot below shows that Safari is using about 500 MB of RAM. A single tab can use more than 100 MB.
I also have a lot of processes running in the background. I have Dropbox, Skydrive, Crashplan running. There over 100 using more than 1 MB of memory.
I replaced the RAM and I now have 8 GB. Starting Safari put me over 4.0 GB. With Evernote, Messages, Safari, and Mailplane running I show 6.8 GB of RAM being used. Clearly, I needed the increased memory. I hope that 8 GB is enough.