When you say the software can't keep up, what is it you're seeing?
ArtRage is a very different application to Sketchbook in several ways. Firstly, ArtRage uses about 3 times the amount of memory to represent each pixel in your canvas. As well as colour information there is the depth of paint, the wetness of the paint, the shine of the paint, and how reflective it is. As we're compositing against a separate paper texture we also need to keep the alpha of the paint at each pixel.
Once the paint is applied to the canvas it then has to be run through a 3D lighting renderer to get the natural look and feel of the paint. The 3D rendering is very CPU intensive.
However, we do a number of things to keep the painting process responsive. When you're doing a long fast stroke we reduce the number of updates - we can do more efficient rendering if we assume we're going to render a bigger chunk, and do a large update, than if we constantly render small chunks and update constantly. So if it appears on a fast stroke the update lags your brush slightly, it could be that we're just increasing the granularity of updates to get faster rendering performance.
Ignore the CPU usage indicator - it's not a true indication of the way ArtRage works. During a stroke we reserve all the idle processing unless something else wants it - it doesn't mean we're actually using it. (This will likely change in a future version of ArtRage as it does lead to confusion).
However if you're seeing a long delay between stroke and update it could be something else.
2GB is a decent amount of memory - it sounds like your machine specs are completely adequate for ArtRage. Hell, we get good performance on a 800MHz compuer with 1GB Ram.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"