Depends on the RAM. ValueRAM I wouldn't go above .1 of it's normal spec which is generally 2.6
Some high performance RAM like the ones in your sig are rated at 2.7 or 2.75 so you'd need to bump it to 2.8 possibly for more stability.
However it's much easier to fry your RAM than your CPU via adding voltage.
The heat put out from upping the Dimm voltage really doesnt show on the RAM unless you are hitting high overclocks and voltages meaning like a dimm voltage of 3 or higher which I suggest you not try and do until you are way more familiar with overclocking.
Generally you'd only need 3 volts in trying to get DDR400 RAM up to like DDR500 equivelant which would mean putting the FSB from 200 to 250 in this case.
If you think it may be your RAM then there are a couple steps you should try before raising the voltage as that should be your last step in trying to increase stability.
Get a program called memtest86+ just google the name and download it. Put it on a bootable CD or floppy.
Now make sure the CD or Floppy is first in the boot order and let the comp boot from it.
It'll then test your memory with 7 different tests. For heavy RAM overclocking I suggest you let it run the extended tests atleast a couple of times.
See if your memory gives you any errors.
If it doesn't then you can rule out the fact that the memory is causing instabilities.
If you do get some errors generally raising the cycle time from 6 to 7 or possibly even 8 and 'loosening' up the RAM like that will allow for stability. Again you'd want to test the RAM after loosening the timings and see if the errors still persist.
Generally you won't need a voltage boost to the dimm unless you are pushing around 215-220 FSB, but like with everything, all systems and RAM sticks differ so you may need to do it at 210 or even 205 but I highly doubt it.
Upon higher overclocking loosening the other timings are needed as well.
Like if you wanted to hit 230 you'd probably have to be like 2.8-2.9 dimm voltage with timings like 2.5-3-3-9 or even possibly 2.5-4-4-9 or a combinatio of 2.5-3-4-8 of course it would require a lot of testing, trial and error and such.