Temporarily disabling RAM to mimic a lower spec machine? I've got two 2009 Macbooks. One has 4 gigs of RAM and runs os 10.10; one has 2 gigs and runs os 10.5.

I'm considering upgrading the latter machine to Yosemite (10.10), but I'm worried that it will run slowly with so little RAM. In order to get a sense of what the performance would be like, I'm wondering if there's some way I could "turn off" half the RAM in the higher spec machine. If I did this, I could get a sense of how Yosemite would run on the 2 gig machine before actually doing the upgrade.

Is there some way to temporarily disable RAM without physically removing the chip?
There's no need to take out RAM, create a RAM disk or use a VM.  Simply boot the OS using the maxmem= boot flag value that's been created for this purpose and been around for decades.

Simply open Terminal as a sudoer and enter

sudo nano /Library/Preferences/SystemConfiguration/com.apple.Boot.plist


After entering your password change 

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs$
&lt;plist version="1.0"&gt;
&lt;dict&gt;
        &lt;key&gt;Kernel Flags&lt;/key&gt;
        &lt;string&gt;&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;


to

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs$
&lt;plist version="1.0"&gt;
&lt;dict&gt;
        &lt;key&gt;Kernel Flags&lt;/key&gt;
        &lt;string&gt;maxmem=2048&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;


and write the changes to disk with ctrlo and quit nano with ctrlx

Restart your Mac to apply the changes.

To revert the changes remove 'maxmem=2048' with nano again.