Hi and welcome to the forum.
Yes it's true that there are empty CD's which can hold more than 700MB/80 minutes. 90 minute ones are available anywhere (actually they are currently easier to find than 74 minute ones!), while 99 minutes' ones are not so common, but still not difficult to find. Virtually all current burners cancope with a 90 minute Cd, and the big majority of them with a 99 minute one as well. Most burning applications (you can exclude Crooxio CD/DVD Cremator) can burn those CD's.
That thing you describe isn't called "overclocking", but rather "overburning", which has to be enabled for burning any CD longer than 80 minutes. The amount of extra data you can put in depends on the media you use, and to a lesser degree to the software.
Putting 750 MB of data on a 700MB disk is fairly easy (in fact you can put more than 800!), but for doing so you must use a noncompliant burning MODE, which may render your CD unreadable. More about it
here
You will find plenty of valuable info on the subject at
CDRFAQ infosite