I am not a mac person, but the system you have is what is called a 'cusp' system.  It is near the change over point between 32 & 64 bit.  Now odds are it's a 32 bit system.  I think you have max'ed your RAM out, so there is little you can do to increase your speed.

As for Vue.  Processors, Processors, Processors.   I have a quad 2.6 GHz Xeon that I run Vue 5 on.  The system has 8 gigs of ram.  Vue flys through renders, but it's not the amount of RAM.  It helps, but rendering is all CPU cycles.  I have a Sony Laptop 3.3 GHz HT with 2 gigs of RAM.  The Quad blows it away at render time.

This has been my experience on the PC, but I will lay odds that it applies to the Mac as well.