You probably won't like this answer but this is the kind of question that is almost hopeless to answer. Some rought guidelines can be given, but you circumstances are your own.

The best thing you can do a set up your containers with some "reasonable" defaults and be ready to finetune it. The downside to this is that it will take you a little bit of time and effort, but on the plus side you gain quite a bit of knowledge and experience along the way. And that increases your value (as an OpenERP implementer).

You've got enough disk space. You might want to leave quite a bit of it unused (with LVM you can always attach it later). Your RAM sounds a bit on the low side perhaps (but again, it depends). You'll need to monitor your memeory usage and be ready to pop out and upgrade your core if/when necessary.

Finally I don't think separating OpenERP from postgresql makes it any easier to "control and manage" (your words) the two systems. Rather it gives you the ability to scale the servers separately, so that you can give resources to the server that needs them the most. It can also have security benefits, if that matters to you and if you set the systems up appropriately.