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I run an Imac 21.5 3.2 i3 with 12gb of ram

Started off with 4gb but too slow when batch processing.

12gb really flies and with ram so cheap these days (8gb from crucial for £70) it is definately worth it
 
I run a 3.5 year old iMac extreme with 4 GB of memory, it runs Aperture 3 perfectly, Some of my RAW files to over 100MB. Aperture seems to used about 1.7 - 1.8 GB of ram when running.

I also have a MacBook Air 11 Ultimate (4GB) and it runs Aperture 3 also now problem, but it is a little slower on the processing due to it being 1.6 C2D.

To speed AP3 up a will want my next desktop to have SSD drives, and faster bus, plus a quad core processor, seeing AP3 can get processor hungry.
 
RAM is less important than time. You need time to deal with the demons that reside in APP 3. When you get them tamed, then worry about RAM.
 
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