2.6 wont do anything to hurt it at all
asus even overvotls on ram default on most, if not all, boards
150fsb on pc2700 is still underclocked (if you have it at a 1:1 ratio that is...) and should be fine
sometimes its needed however....so if you need it, up to 2.7 is ok in my books
2.8 is getting a bit extreme (not that it stops some...including myself hehe)
any higher often isnt availabke....but if it is, some choose to make use of it to "test"
i dunno about long term tho....