I've been quite a few times now and whilst you may feel you have become an expert on GT4, one thing you don't experience with the game is the way the track dips up and down, curves and also the g's that you can experience - the Ring is a dangerous place, so I would be extremely careful and take your time, especially if you have never been before!
as Andrew says above, the link he's posted will show you what can happen if you get it a little bit wrong
I have been when it was damp and the track gets extremely greasy, which caused my car to spin - luckily it was quiet and there was nothing around, and the car didn't get onto the grass
just take your time and enjoy the experience
when we go, we usually go Speed Ferries from Dover to Boulogne and it's about a 4 ish hour drive from there - they tend to be cheaper than the tunnel and it's only about 10 mins longer too! tickets cost around 21 euros for a single lap, but you can buy tickets for 4 laps, 8 laps, etc which work out cheaper - have a look at this site for all the info Ben Lovejoy's Nurburgring Nordschleife website