IMO 4x DVD speed is the max you should use in general.
On a side note:
Some burners can get away with burning much faster than others while still maintaining quality. My previous DVD burners (NEC 1300A, Sony DRU-150A) both had inconsistent burn quality from one disc in a spindle to the next that was linked somewhat to the speed. I didn't want to waste anymore discs so I did some investigating and heard a lot of praise for the Pioneer DVR-11XD drives and Pioneer drives in general. I ended up getting a DVR-216D (SATA instead of IDE like the 116D is). I have burnt everything @ 4x on it and the discs read beautifully on every drive I've checked. Not to mention it is able to read discs that are 7 years old and burnt on a crappy burner that couldn't even read the discs it wrote some of the time.
Older burner/older firmware: try burns at 4x and 8x and see what works best with your burner and media, it's hard to say. The main problems with older burners are not supporting faster burn speeds and not knowing burn strategies for newer media codes. If your burner is really old and really shitty you may need to burn at 2.4x, but if that's the case you should be looking into getting a new burner.
Newer burner: in general, you will get a poor burn if you burn slower than half the rated speed of the media. Do not burn 16x media under 8x unless you have a very good reason for doing so. For 8x media, try 4x and 8x. For 16x media, try 8x, 12x and 16x and see what produces the best result, but any of those speeds should produce a decent result.
Test your burns with DVDInfoPro or Nero CD-DVD Speed/Nero DiscSpeed to find out in excruciating detail just how good (or bad - quality score of '0' is not a good thing
first of all a SLOW burn can be a coaster just as likely as a faster speed burn
as long as you DVDR drive is capable (update all FW for your drives) you should burn at the speed listed on the disc itself.
if you get problems THEN start lowering speed. i use 16x verbs and 8x imation discs, always burning at the disc speed.
Uum, I would say 8x because I burned Guitar hero aerosmith on 4x and the game froze at a certain point in the game, then i burned it again at 8x and it didn't froze at that point(i tried the song 3times on the 4x and at the same point it froze...)