I just installed Dorico on my early 2008 Macbook, OSX 10.11, SSD, 2,4 GHz Intel Core 2 Duo, 4 GB 667 MHz DDR2 SDRAM
It is within the specs, if I understand them right.
Dorico works, but when I playback (in whatever mode) there are quite some flaws and hiccups, already with two instruments.
Activity monitor indicates it should be possible: most of the time it is at 30% (but when loading the sound files 75%).
Would installing 6GB (instead of the current 4GB) be worthwhile? Or should I immediately go for a faster computer? (Rather not, of course).
Or is there another reason it does not work properly maybe?
You could try increasing the buffer size inside the Device Control Panel, accessible via Edit > Device Setup.
But 4GB really is the minimum RAM that we could recommend for use with playback in Dorico, and the more RAM you can add to your Mac, the better.
Unless you really need to cut the cost to a minimum, installing RAM sizes in “powers of two” may give you faster access speeds as well as more memory, and avoid any weird incompatibility problems.
In other words, 8GB (either 2 x 4GB or 4 x 2GB modules - I’m not a MAC user so I don’t know what options your motherboard can take) is likely to be better than 6GB with 3 x 2GB modules. Mixing modules with different sizes (e.g. 1X4 GB and 1X2GB) is often an even worse idea.
I had a MacBook of similar vintage, and upgrading to the bus maximum of 6GB did improve things when running Creative Suite, for example.
You’ve already got an SSD, so the bottleneck’s not there.
You have to put in 4 GB + 2 GB. There are only two slots. If memory serves, putting 2 x 4 in causes massive problems, because the architecture just can’t support it.
However, it is a 9-year-old computer… 
Thank you very much, the three of you, I am going to try first the Device Control Panel.
Indeed the only other possibility would be 4 + 2, the question is indeed if it is worth it.
Increasing the buffer size already did the trick!