There are 2 seperate volatages.. The core voltage and teh I/O volatage.
The core volatage is what is being applied to the cpu. When overclocking upping the volatge may help in added stability. But upping the core volatge also increases the heat from you cpu. For instance my CPU is stable at 896@1.85 volts.. but if i drop it to 1.800 volts i get intermittent lock ups or errors. You want to run you cpu at the lowest voltage to that keeps you stable. This will aid in reducing heat.
The I/O volatge is the power thats being supplied to your periphs(e.g. vid card, modem..etc). Upping this voltage can be very dangerous also, you dont want to fry that new video card do you..? I have mine at 3.50volts because at the default 3.30volts me geforce2 MX(also overclocked to 200core 210mem) isnt completly stable during heavy graphics load(mostly just 3dmark2000, but an occasional lockup in quake3 after a extended play period). In general the geforce2 cards like the higher(3.5) volatge on the via chipsets.