x2603 dell ati radeon 9800 vs geforce 9400 gt

Are you talking about Laptop GPU's or desktop GPU's? The AMD/ATI radeon 9800 is from like 2003 era (and probably an AGP based card) and the nVidia was released around 2008 (PCI-e based card). So with that information (assuming that is what you meant) the 9800 gt is the better card, assuming you need a PCI-e based GPU.

What is the system your putting this GPU into?