Geforce 470 Card in Dell R710 or R5400 - Will it Blend?
Solution 1:
I just googled the Quadro 6000-series to see what it was and my jaw dropped. I understand why you're not keen on forking out for it!
The Windows 7 drivers should install on Windows 2008 just fine, if they are signed. Now that 2008 R2 actually requires a high-end graphics card for some of its fancy new Hyper-V features, you'll find full support (I assume you're using 2008 R2, but you only mentioned 2008, but the drivers will still work).
I suspect that Dell might use it as an excuse for not supporting the installation, so if you have any issues your first point of call would be to remove the graphics card.
Apart from the issue of it being officially "unsupported", you shouldn't have any troubles. Dell have a bit of a history of going out of their way to provide PCI-e expansion (The M610x blade for example) so it would be highly unusual if you had any issues.