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