I think the best investment you can make , if you've been playing a while, is to buy and amp that can make the basic noises you want it to.
I find that people often buy really nice guitars, and neglect their amps...the fact is, even an amazing guitar will sound bad through a bad amp...simple as that.

Since you want metal, why not have a look at an amp with a really nice distortion channel? I have an Ibanez Thermion and the distortion is awesome, but strangely enough, I bought it for it's clean sound.

Take a look around at some amps... sometimes a solid state amp produces distortion that is better suited to metal needs than tube amps.

If I were you I'd get an amp...but if you're set on a pedal, maybe a Visual Sound Jekyll and Hyde? Or even a Digitech Metal Master? Boss Metal Zone? There are numerous distortions out there.