Actually, I'd recommend two--a Digitech Bad Monkey for light to moderate overdrive (blues, less heavy classic rock) and something like a Marshall Guv'nor Plus for more outright distortion. You could get both of these pedals, including shipping, and still have a few bucks left over from your $150 budget. I've been very pleased with the wide range of distortion and tonal control that my Guv'nor gives me--a very flexible distro box. If you want serious metal-style distortion, then look at something like the Boss MT-2 Metal Zone or Digitech Metal Master. These will give you that crushing, heavily mid-scooped death metal distortion.