Thanks for the answers.
1. About overspeeding: Mostly TY, Maxell, SONY, TDK and KIC 4x DVD-R Made in Japan Discs.
2. The problem I am having is that i bought a LG-GSA-4163B on dec. 28th after the sales rep. at the shop told me that the LG drive is better at overspeeding 4x DVD-R than the NEC drives. Well I bought one and soon after installing the drive found out that the LG does not overspeed 4X blancs. So I will return the LG and get an NEC. The next thing is that the timelimit to change the drive is on the 5th of January (not much time left to decide which NEC). Now i dont want to get fooled by an unknowing sales rep. again...
So any further information would be very helpful.