If you have real life experience then you'd probably be fine.  The difference between the 70 and the 83 series is that the 83-series includes simulations in a virtual lab.  From what I've been told by several peers who have taken the 83-640 (it wasn't available when I passed the 70-640) it is not all simulation/lab.  There are a number of simulations/labs, but also a number of the traditional questions.

In my personal opinion it shouldn't make a difference.  The knowledge being tested is the same either way.  The only concern I would have would be if you had no hands-on experience working with AD Domain Services in Server 2008.  But as long as you know your way around the interfaces (which is easily solved by running some Windows 2008 VMs and installing/configuring/managing AD), then the 70-640 prep should be just fine.

________________________________________
CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCTS:Windows Server 2008 R2, Server Virtualization
MCSE:Security 2003
MCITP:Server Administrator
MCITP:Enterprise Administrator  

Thanks for the advice. I think I will just work through the examples in the 70-640 book that I have and use the virtual 2008 servers that I have installed as testboxes.

I tried to sit this exam twice yesterday and both times the test blew up and I never got to finish it. What a waste of time, I was at the test center for almost 4 hours.

Make sure that you report that test center to Microsoft.  There were some early teething issues with the virtualized lab exams, and the performance is highly dependent on the amount of bandwidth available to from the testing center.  One of my colleagues had a similar experience, and the last I heard was the Microsoft was keen to address any issues at testing centers that were specific to the 83-series exams.

You can email Microsoft's Psychometrician, Liberty Munson, at liberty.munson@microsoft.com.

________________________________________
CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCTS:Windows Server 2008 R2, Server Virtualization
MCSE:Security 2003
MCITP:Server Administrator
MCITP:Enterprise Administrator