The only difference is the XTX is all together a faster card , its not because it is ATI or Nvidia , your not using the same grade of card , thats why it seems faster , makes perfect sense , show me where the 2 cards are = and then we will say well it is ATI that was the difference not the speed of the card.
In terms of Vista UI, with my 7800GTX there would be some lag between clicking and dragging and the UI responding. When windows are moving it would "feel" like I was dragging paper of adhesive tape or pulling a sheet out from between two books.
With the ATI it's click, drag, stop as I click, drag and stop moving the mouse. It's somewhat hard to describe as it wasn't exctl sluggish or unresponsive with the nvidia card. It just wasn't as responsive as XP's interface was. With the ATI card it is.
It is much improved but I can't put my finger on anything specific.
Whether its down to physically faster hardware, better drivers or something else. Vista is much improved with the ATI card over the nvidia card.