I’m not sure which machines started distributing multiple cores well, but I can at least say SX3 did a pretty good job of it. I was running a dual core system with a Delta 1010 at 128 (3ms Latency). I used a large amount of plugins, but not a lot of VSTi’s and it handled the distribution of power well.
3.25 Gigs with your current setup is pretty much in the standard range with 4 gigs Ram installed and no “switches”. My XP system currently recognizes 3.33 Gigs.
If I remember correctly, there was an issue with Cubase 5 using more resources if you created 1 instrument channel, and multiple midi tracks assigned to that instrument… than if you just created multiple instrument channels. Depending on your FX though, it’s pretty straight forward. Each effect = slight increase in CPU.
With your system though, you should be able to get over the century mark with audio tracks. No offense, it just sounds like your version of Cubase is getting a little long in the tooth, and is just not handling multi-processing the way it should.
If you never checked this out, take a look. Very informative.