I have two machines, both 2.4 GHz quad core machines. One has 4GB RAM, the other 8GB RAM.
Which server should be Web Front End, which should be Database server?
There is no single magic answer for this.
A webserver can run very well on tiny hardware if all it does is serve static pages to a few users. If it needs to serve hundreds of simultanious connections and do some server side magic (e.g. accept uploadd images, convert them locally to a identicsal format and display those) then it needs bigger hardware.
The same is true for the DB server. Without more information than you provided you will not get a useful answer.