Here at work, we have one instance per application, for a total of 6 (small) instances running on our production database server (Sun Enterprise 420R, 2 CPU, 2G RAM). We've had zero problems running this setup, and it's easy to add additional instances, assuming the server has the available resources (you may need to bounce the listener to pickup the new instance). Though, running multiple instances on the same box will require you to be careful when sizing your SGA, but you've got lots of RAM. Make sure you standardize on your filesystem layout for all instances. And don't forget to update your oratab file, if you're manually creating instances. You really shouldn't have any problems with this setup.
