[My DAW: AMD2 5000+ (2x 2.6 Ghz), 4Gb RAM, 2x 1.5TB HD, WinXP. Mostly used for guitar and drum-machine VSTi work]

I've been using Cubase SX for a while now. And for some reason it's been really dragging recently, stuttering and struggling.
Do CPU's get old and tired?

I'd started playing with Sonar7 (a friend's loan), and found it to be infinitely more stable than CubaseSX. much less stuttering/glitches/etc.
... Dont know if it's more tuned into my AMD2 5000 dual processor.

SO, i'm planning to go through the tedious process of formatting and reinstalling WInXP on a fresh and stable sytem, and need to consider the investment in either Sonar7 or Cubase5. So went to the store, checked out their DAW with Sonar7 (or mightve been 8!) and Cubase5, and running Guitar Rig 4, checked out the task manager to see what it was saying:

Cubase5, 44.1kHz, 5ms latency, CPU usage: 11-16%. Ram usage: 288mb.

Sonar7, 44.1kHz, 5mS latency, CPU usage: 17-29%, Ram usage: 238mb.

The RAM and CPU usage went UP with the higher Bit depth and higher Sample Rate. BUT, there seemed to be less stuttering with these higher sample rates?
Accordingly, Sonar, despite using more CPU seemed to stutter less.

Can anyone make any sense of this? Or am i missing the point?