Using a second server as a MySql server is (almost) always a good idea if you can afford it..
But again, the server specs really depends on what kind of traffic you expect so it's really hard to make any recommendations without knowing that..
The server I'm using here is a Intel Xeon 3060 Conroe 2,4Ghz w/2Gb ram and Sata-2 HDDs, and it can cope quite well with 40,000-60,000 daily visitors in total on the sites I'm hosting here.