Another option between an iMac and Mac Pro would be to get a quad - you just configure a MacPro and downgrade the to quad cores (saves $500).  The render times are simply related to the number of cores you have working - so the more the merrier.

Operation though would be pretty much the same on any machine.  So, if you go off to sleep and render, it wouldn't really matter which machine you're using unless you're doing very large scenes. Or as in my case, I like to use very small render previews which run quickly with all 8-cores.  An iMac wouldn't really be much faster than what you already have though except that you could use Hyperview to render on both machines.

btw, 4G RAM should be adequate.  I have 16G as I also do music.

Also, the iMac is a cost-effective choice as it comes with a nice 24-inch monitor.  If you get a Mac Pro though, consider getting a DELL monitor instead.  I have a DELL 2408 which is half the price of an Apple and its really great.

Greg