Hi David,
what seemed good "validation" approach to me is to have one PC with card from ATI/AMD and one with card by NVIDIA.
The NVIDIA cards are maybe better for enthusiasts, because they allow to go "over the specs" in some things, but ATi/AMD cards are more strict on other side, so it is "safer" to develop on ATi and then do tests on NVIDIA, if you develop apps for wide range of customers.
Petr