Since you mention Thunderbolt, I'm assuming you have the new black cylinder Mac Pro.
If that's the case, it has an HDMI output right on it. You would just get an HDMI cable and hook it up.
Besides that, you can use a Thunderbolt cable and go from the Mac Pro's Thunderbolt port to the Mini-Display port on the Dell. Thunderbolt carries the Displayport protocol.
And if you didn't want to do that, you could also just get a Mini-Displayport to Displayport cable, and go from the Thunderbolt port of the Mac Pro to the regular Displayport on the Dell. This basically is the same thing as the previous method, just with a different cable.
If you have the older Mac Pros, then you would just use the Mini-Displayport to Displayport cable and you're good to go.
No matter what, you don't need to mess with DVI at all. And all of those methods will give you full resolution and quality.
If you use the Minidisplayport to Displayport cable, you can find it here:
http://www.bhphotovideo.com/c/produc...splayPort.html
(they make longer ones too).
But as mentioned, if it's the new black Mac Pro, the simplest way is just a standard HDMI cable.