I have a customer who is running OpenERP on a Ubuntu Box (running in a virtual machine) with 1Gb RAM. So far so good... has over 1,000 products and 3 users.

The only problem was not hardware related, was PostgreSQL related. And it was not a problem at all, but you should take a look at the vacuum process with PostgreSQL

I wouldn't worry too much, I've run ours on many different configurations, including small ones (Windows XP SP3 with 2GB Ram) and didn't experience any particular issues.

People have actually successfully installed OpenERP on Raspberry PI units - of course you wouldn't want to run a production installation inside one, but it does give a nice reference of how little OpenERP requires from hardware.