Mr Torture, I have a 10 year old budget Medion desktop that runs perfect on high sample rates, even at the lowest buffer size.
Maybe it's not the computer itself, or just 1 component that's causing the prob.
For a start I would disable '64 bit engine' and check settings in Sonar, fe. try cache reading en/disable, etc.
Also you could dowload a free latencymon software tool, that might show you what the problem is.