Well, technically, ANYTHING over 650MB is overburning. CD-Rs are rated for 650MB/74 minutes, but manufacturers have found ways to allow more data to be put on the CDs. There's a "buffer" space at the end of a CD that isn't supposed to be used, but can be to allow more data to fit on the CD. That buffer area was meant to be reserved for the "lead-out" data, but sometimes you can use the extra space for normal data.
700 and 800MB CDs have become commonplace, but they don't conform to the CD standard of 74 minutes/650MB, and can sometimes have errors if you write over 650MB of data to them.
In a nutshell, the most usable space you'll get out of a CD-R is 650MB or 74 minutes of audio (minus the dead space between tracks, lead-in, lead-out). You CAN burn more data onto a CD, but you may end up with errors and the Mac platform doesn't provide many tools for doing so. Some CD-R manufacturers even advertise their CD-Rs as being able to hold more MB/more minutes than a regular 650MB/74 minute CD-R, but finding the tools to actually burn that much data on the Macintosh is tough.
Here's an article on CD-R sizes and "overburning:"
http://users.forthnet.gr/ath/axatis/...verburning.htm