It's not so much Cubase, it's having a ton of applications running at once that bogs it down. Like, Thoth takes up a ton of space to start out with, then gradually keeps taking up more as it downloads files.
If you have Safari and Entourage going at the same time and then add in MSWord, Split & Concat, MacPar Deluxe... plus all the extensions like Time Machine, CopyPaste, Default Folder, Growl, TypeIt4Me, MagicMenu, a menubar calendar and OpenSpaces it only leaves you with about 600MB of RAM left over.
Not to mention watching DVD's and having Big Brother running on RealPlayer at the same time.
Once you start copying files or downloading using the browser, it makes it even worse and it's like going back to my old Mac G3 again it's so slow.
Then all it takes is one more application like iMovie and things start crashing.

I'm just trying to push it too hard for the amount of RAM it's got. Unfortunately, CanadaRam says they won't have anything in stock until mid March.