I have some Samsung chips that crap out at 2.85v and OC best at around 2.7v, but on the other hand I have some Winbond UTT CH-5 that doesn't even get started till 3.2v and has handled that voltage nearly 24/7 for 18 months without heatspreaders and only the air flow from my zalman CPU fan to cool it.
Newer and higher density memory tends to handle high voltages (over 2.9 or so, for DDR-1) less well. Model and air flow also makes a big difference.
Some of them. Generally, the more memory you cram into the same number of chips (ie 16 on a typical double-sided dimm) the smaller the manufacturing process is for the memory cells. Smaller processes require, and tolerate, less voltage.
Now there are still some 1GB DDR dimms out there that you can put some very high volts through, but in most cases they will need better cooling than comparable 512MB sticks.