overburn should be on by default, it does nothing if the disc image is small enough for the disc (so it can't do any harm) but if a disc image is more than what a CD can normally store then overburn kicks in, but if overburn is not enabled the burn will simply stop and the disc will not be closed correctly giving a coaster CD-R (that does not give a lot more space by the way just few MB) anything that targets 99min CD-R's though needs overburn on regardless as anything more than what a standard 80min CD-R can store must be overburned
This may be an issue with your dreamcast though see http://www.epforums.org/showthread.php?23311 for more info
P.S. sorry for the late reply
I missed this post totally
P.P.S. give me a list of none working games as the other issue may be that they are not built correctly I know we currently have though I hope to help in fixing this, a bunch of none working Dreamcast cdi images on our main server, Utopia should not be required for a fully Mil-CD compatible Dreamcast that is all PAL dreamcasts and NTSC-U Dreamcasts dated Model 0 and 1 and model 2 dated October 2000 and earlier there are a few Dreamcasts mostly NTSC-U versions that only work with data/data Mil-CD's, this kinda sucks for any games with CDDA tracks though