if you aren't running many scripts then really a fairly basic dedicated setup will do plenty fine. I have two sites on a dedicated box now which essentially the whole site is php/MySQL. Each site does anywhere from 13-20gb of bandwidth a month with millions of hits (page views, not individual visitors)
It is a 1.2ghz pIII with a gig of ram. I must say it runs great, and at a great price. So, take that how you want to, as you can find servers with less ram and less powerful processors that could suit your needs, but sometimes the very small price difference between the bottom line and something in the middle makes it worth to buy a bit more powerful machine. This way you always leave yourself room for growth as well.