The jump from 3.8 to 4.0Ghz is minimal. Your SuperPi 1M times decrease's maybe less than a second and gaming performace is negligible. 4Ghz is more for personal satisfaction, even if it is only a boot and bench .
I'm comfortable running mine at 1.45v for the 24/7 4.2Ghz (seems a bad chip in my eyes), it's not recommended but to be honest it's my money so i'll do as i wish.
Regarding the vdroop, I found my board P5E to have quite a high vdroop, after google searching a pencil mode for vdroop and with great risk to my computer I performed the act and can say that it has worked for me.
I've read things that say both, some say leave it on some say turn it off and I really haven't found a difference between the two. With it on the system is stable at 1.36 and with it off it is stable at 1.4 (1.36 after droop).