Just a thought:

Tom Kyte (http://asktom.oracle.com) often says one-box-one-instance. That way you'll reduce the wasted disk space and memory associated with many instances on one box. That would alleviate some of the problems of them sharing the same machine provided you structure your security and working practises correctly.

Lots of instances on one box is fine provided the box is sized properly. I would consider using RAC because it works well and it allows for quick scaling should the need arise.

Cheers

Tim...

PS. Nothing wrong with not coming from an IT background. I did a PhD in genetic engineering before I became a computer geek.