Dick,

its been a really long time since I have used ATI cards on any Linux system I have built or installed for other people.  I would love to tell you that one is better than the other, however, my only experience has been with Nvidia.

What I can tell you about Nvidia is that they are really aggressive in keeping Linux support for their cards.  The features that work in "other" operating systems will also work in Linux.  They even have a graphical configuration application that works with their cards for enabling and disabling advanced settings.

Personally, over the 15 years of using linux, anytime I build or consider building a new system I don't look at any other brands.

My laptop, although now a year old, was bought the day Windows Vista came out and it worked flawlessly with the Nvidia drivers for Linux.  My desktop, which has a 7300GT PCI-E card was purchased the day it came out on the market.  It took about a week before Nvidia had the required modules for a 64 bit operating system.  However, it worked fine using a previous driver compiled for 32 bit.

You may want to see my Linux Hardware blog on my web site at http://www.edwiget.name  You will notice that although I have buying "hot-off-the-shelf" hardware lately, it has all worked with Linux with very minimal configuration.  I have spoken with others who have stated the same thing.  What this leads me to believe is that you will likely have the same experience with the brand of video card you purchase.  You may want to specifically try to find others using the cards you are wanting to purchase first, just to be sure.