I'm no techie but I'd guess that with the change, you are optimizing your RAM & CPU for your system cache which your charting program uses to store the incoming live data for drawing charts. I think your hardware is fine. 15% CPU usage is about what I get with the same machine running QuoteTracker, with spikes to 40-50% during high volume periods. It just purrs along beautifully although the 2Gig machine is more than 4 years old.
http://techrepublic.com.com/5100-10878-5034438.html In systems with less RAM, you need to devote as much RAM as possible to just running Windows and your applications. For a server or a desktop with a lot of RAM, however, choosing the System Cache setting will yield better performance. When set to System Cache, the system will use most of the available RAM as a disk cache, which can result in major performance improvements on systems that depend on disk I/O.