If you're looking for the cheapest possible way of getting the metal sound, just hit the guitar shop and find yourself a kicking distortion pedal (or two, if you plan on using different timbres of distortion, or playing lead as well as rhythm in a band situation). Ask the store staff whether you can try out a few different models, and pick the one/s you like the most. But if you've got dough to spare and you're in for the long haul, do as Schmange suggested and get yourself a nice big modelling processor like the Boss GT-8, the Digitech RP350 or the Vox Tonelab LE. Also, make sure your amp is up to the task, ie. it has a dedicated high-gain or overdrive channel - if it's a single-channel job with a gain boost, or a completely clean amp like a bass amp, you may get some excessive feedback happening.