In terms of drivers, it should rarely matter. Let me demonstrate;
I go to the download page
Drivers & Software
And for example I select an operating system, Vista 32, then the family, Radeon, and the type, 4800 series; it recommends ATI Catalyst display driver 9.1 for Vista (32 bit)
If I chose the 4870X2, it takes me to a different page, but still offers the Catalyst 9.1 package. These drivers have been WHQL Certified, but that doesn't mean it's not the same driver on the main page. Windows Hardware Quality Labs testing can be handy for drivers that tend to be buggy, but locked in competition, both ATI and nVidia are directly answerable to a vocal army of nerds and if they make a driver that doesn't work, it's a big publicity storm.
If I choose the 4670... ATI Catalyst 9.1 again.
If I choose the 3870... ATI Catalyst 9.1!
What about an old 9700... I had one of these 3 and a half years ago, surely support doesn't go back that far... no wait, ATI Catalyst 9.1!
The driver packages by nVidia and ATI (ForceWare and Catalyst, respectively) are designed to be an all-in-one package that avoids confusion. Most videocards are not different enough to require a wholly new driver, with new cards never throwing away all of the features of the last generation. As a result, it's only if you're working with an unbelievably old card or a unusual mobile device that you might need to look for a specific driver.