Shenmue Dojo

Looks fucking amazing.

and how do I overclock my dc?

Wish I knew bro. Look here.

http://www.racketboy.com/retro/overclock-your-dreamcast

And here.

http://dcemulation.org/?title=Overclocking

As an expert on overclocking regular ol' PC's I can tell you you absolutely need to know what you're doing if you're going to overclock a Dreamcast. These consoles (especially the DC) are designed with cost and form factor in mind, and the second you add any volts to the CPU and or GPU the thing is going to heat up drastically. You could upgrade the thermal pads to some high quality paste, but even then I don't think you'd have sufficient air flow (that little fan is so sad) or surface area on the heatsink. The only viable way I can think of to do this would be to run it outside of the plastic case and add some kind of DIY cooling solution.

This stuff isn't cheap, but it's the best out there - http://www.amazon.com/Coollaboratory-Liquid-Thermal-Interface-Material/dp/B0039RY3MM

I had an overclocked Dreamcast for a while, a couple of years ago. I had it run with 240 Mhz, wich is 120% speed.

It did an amazing job at delivering Shenmue II with less slowdowns. But one day it just wouldn't start anymore.
I don't think the CPU, RAM or GPU gave up but rather the crystal I used or it's connection came loose. I never really looked into the matter again since. Maybe I should consider it again at some point.

Looks incredible dude