Usually you can go to 2.9V without any issues. Anything over that and you may want to consider heat spreaders on the RAM, or RAM designed for higher voltages.
I have some Corsair Value Select PC3200. It won't run 200MHz @ 2.6V, but the Corsair support website even says to bump up the voltage a bit before worrying about it.
Give 2.7V or 2.8V a try, run some Memtest86, and see what happens.