Althought I am not terribly familar with Cubase I would suggest to check their user forums and do a search, I am sure that someone has had a similar issue.

Wise

I'm guessing you're already well aware of this part, but just in case... this sure sounds like the soundcard buffer setting, where buffer size is inversely related to latency. So on my setup I change the soundcard's buffer size until the latency (as reported by Cubase) is under 20 ms or so. This is usually more than enough to ensure no discernable delay between key and sound. At least, not from the soundcard. I presume that a slow or overloaded CPU/memory prob could cause probs, though I haven't had that experience so can't comment. But looks like you're in good shape given your CPU speed. As long as you don't overload your RAM with lots of VST's simultaneously loaded (or use the "freeze" feature (I think) to deactivate those you're not using in order to free up memory) you should be fine in that department.

Have you tried this already?