Yay!

My limited experience and that of others I know (perhaps a dozen of us that geekfest (err, LAN party?) 3 or 4 times a year is that no one has ever hurt their computer by transporting it in the car. And almost all of us have AMD's. I really wouldn't worry about it. (Yeah, famous last words, hope I didn't jinx myself). If you're really worried, what about sliding a piece of stiff non-cunductive non-absorbent closed cell foam underneath it. So even if it does want to bounce it has no where to go? But seriously, even with a lot of weight, it's spread around really well...

BTW I have a Thermalright AX-7 and 80mm fan, maybe not as big as yours but somewhat heavy as well (if that lends any merit to my successes moving this PC everywhere). http://www.svc.com/thaxsoco.html