Guys, thank you all so much. Well, I've tried different CDs from the same pack but all show the same error. Finder burning is actually worse than Toast as it reports the disc to have '670MB' ! FirerStarter doesn't even recognize my drive (a Pioneer 105!). Funny thing is, I once managed to burn a 800MB CD... About the disk image. Funny again, I created one at 800MB, but couldn't even copy the two files I wanted to burn although they are both about 386MB... The file sizes, however, seem to be ok.
And anyway, Toast still wrongly reports the size of the CDs.
Over burning: Well, I managed to overburn some in Toast, but, AFAIK, this is not about overburning (or maybe I got the terminology wrong): The CDs are actually advertised as 90min./800MB. I don't know about other countries but here in Germany those aren't that uncommon...
@Fryke: I'll get a different lot of discs soon and will see how I fare.