Overclocking legend Shamino, who is working as a technical consultant for Foxconn's Quantum Force brand, has posted a picture tutorial of how to mod your HD 3870 X2. The only catch is that you need to have a VCO PLL driver bug-free HD 3870 X2, or you are stuck at 860MHz no matter what you do.

Those with VCO PLL driver bug-free cards and some soldering skills can jump to Vmod their HD 3870 X2, and try to take that 3DMark throne or at least try to come close. DAAMIT might not like this, but the community certainly does.

You can find the tutorial here, and as Shamino said, happy overclocking.