I've got a T9800 (2.93Ghz) and the lowest voltage value CoolBook shows is .9000V, so that is as low as I went (and CoolBook allows me to). I reckon CoolBook will show an ever lower value for your T9800, and it's perfectly safe to (try to) use that voltage value. There's some user on this forum that also has a T9800 if I'm not mistaken, and he is able to run every clock frequency at .875V.
So give it a try Dan, and make sure you have adequate backups of your data before running CPU test!