I have a powerbook G4 1.67 and I had opened Logic Pro and Rewired Reason. In Logic pro, I had loaded the EWQLOS Gold multi instrument and about 6 instruments under that and in Reason, I had the Drum Kits 2 open with one (24 bit) drum kit loaded. I also had 2 EXS24 instruments open with one instrument loaded in each. With this, the Powerbook was reaching the max for both CPU and RAM (1.5Gig). It didn't freeze or stop working but I started to see things were slowing down. I'm pretty sure I could record audio tracks on top of it but the point is, I had not opened enough VST instruments or racks in Reason. I usually need more instruments than that.
I know I could Freeze some tracks but I wouldn't want to do that when the ideas are flowing. After seeing this, I have decided to go for a G5 Powermac. Would G5 2.0 dualcore be fast enough? How many more VSTs could I open with it?
This doesn't exactly answer your question, but you may want to see what (if any) Intel models are coming out in the near future. Sometime in January is when the news releases etc come. Maybe there will be something better/cheaper coming out in the near futue?
But to answer your question, I would imagine dual processors (if your software is optimized for them) and a lot of RAM in the G5 would greatly increase your track, VST, etc capacity although I wouldn't be able to back that up with specific numbers.