Aperture now runs at 64bit. But you may want to think twice to run it at full.
Your Mac boots normally at 32Bit, Aperture will start in 64. To take full advantage of Aperture 3, and allocate more memory for the app, you will need to boot in 64Bit mode. Check the topic:
Aperture 3 is in fact running at 64Bit mode, by checking the System Profiler. (Software-Applications)
There is one way too start Aperture in 32Bit mode. By checking the box on the image below:
Now, the memory adressed on a MacBook Pro 13″ with 2Gb of Ram for Aperture, is clearly not enough…
Now you have three options, check that box and put it at 32Bit mode again, buy more memory or boot your Mac at 64Bit and, well, buy more memory anyway..;)