The "ATI 3780 Mac & PC Edition" is probably the best choice. On paper it's similar in performance to the 8800 and it's priced about the same - however Color and Motion are optimized for ATI cards, not Nvidia, so those apps benchmark much better.
I recently made the mistake of buying an 8800. Because the 3780 is ATI branded, not Apple, it flew under my radar.
your old ATI card will work fine if you put it in your new machine... I just did this with 4 machines in our office. I bought 4 new workstations and gave them to our lead editors... but had issues with new ATI HD 2600 XT cards and FCPs defocus plugin. It would give us Graphic Card errors in FCP when we went to render out a sequence. Now I'm just researching if I should buy the ATI 3780 or go with what I know to work well the ATI 1900s... I'm hoping that the 3780 would be faster the 1900... I need to buy 4 of them.