Did a little test today to see how my CPU was functioning during FFXI play, just using Task Manager to monitor CPU and RAM usage. CPU was usually from 55-64% except for one random jump(during about a 15 min period) up to 75%, I say random because it was only for a second. Opening the equipment menu while running around and doing various things didn't seem to make a difference, large crowds of people made the biggest impact.
To see how far it would push it to multitask a little bit I opened up google chrome and put it on a youtube video and then windows media player and put it on a song with one of those colorful, spinny displays and then ran around in the aetheryte camp through a large crowd. CPU Usage was stable at about 70%, sometimes jumping to 75%.
Only really took note of the highest RAM usage, which was 2.12GB when I had Chrome, WMP, and FFXIV open.
CPU: AMD Phenom X4 9650 OC'd to 2.7GHz
RAM: 4GB G. Skill DDR2 PC2-8500 at 1080MHz