A completely transformed internet browsing experience.
An old 2007 white Macbook went from 'unusable' with 2Gb ram, to 'effortless' with OWC 2x2Gb. It had been running out of memory on loading a single newspaper webpage - thank you the 'Daily Heil', er, Mail - and filling the very small free 2Gb hard disk space (my fault, partly, for choosing too small a partition size). Now it nearly always has 1Gb ram free on Activity Monitor no matter how many windows, tabs and applications are open. And the hard disk free space is unused. Only sorry I persevered with 2Gb ram 'unusable' performance for, ER, 3 years. Doh! So don't do what I did - upgrade today! ; )
Interestingly, the question of whether 4Gb ram would be sufficient ($33) or 6Gb was needed ($110) is resoundingly answered as '4Gb is plenty' (for internet browsing only, with OS X 10.6 Snow Leopard).
Browser upgrades and newspaper webpage changes previously increased the load on the system from 'trivial' - 2Gb of ram is plenty - to 'ridiculous' - 2 Gb ram is unusable. With 4Gb ram - and OSX 10.6 - it's back to a 'trivial' load on a 2007 white Macbook. Pageouts remain at 0, almost entirely. Previously they were, er, rather large - 1 Gb plus in an hour. CPU use now is heavy, but fine. Much heavier than previously. Disk use seems to now be 4Mb/s versus as high as 20Mb/s with 2Gb ram. (Yes, it is on a USB 2.0 external HD. Don't ask!) All in all a great example of going from ram bottlenecking performance at 2Gb to a well balanced system at 4Gb.
Looking forward to testing a 2009 white macbook with 2 or 4Gb ram and HD or SSD and comparing the difference in Activity Monitor between OS X 10.6 Snow Leopard and OS X 10.11 El Capitan for the same load.