Dell's top of the range XPS 710 model still doesn't do Vista. This $5,500 worth of PC still cannot handle Windows Vista and you can't even buy it with Vista. It is Windows XP media center edition or nothing.
Things get even more complicated as you cannot get this beast without an operating system. Dell claims that it is still evaluating drivers and Vista compatibility and it doesn't want to bundle Vista until it is certain that the drivers are stable and performing well. Let me quote the company's statement.
" Windows Vista Coming Soon
While Dell continues to validate the compatibility of Windows Vista with popular gaming software & accessories, we will continue to offer XPS 710 customers Windows® XP operating systems. "
Dell can get this water cooled system and the QX6800 quad core up to 3.46 GHz, but it cannot get Vista right. At least you will get a PhysX card with the machine, but you won't get much use out of it.
You can read more here.