Sounds like you're after the "min-height" property as this will stretch downwards if/when the content inside your div expands. Otherwise it will stay at the height you specify.
A word of warning though, this property is not supported in IE 5/6 so you'll need to do the necessaries to "emulate" it in those browsers. There are plenty of links on Google for fixing this.
Hope that helps!
- Mike
Edit: Looks like Marc beat me to it
