First: ATI X1900XT 256MB PCIe
Second: NVidia GF 7950GT 512MB PCIe
What are the beneffits of ATI and PCI?
In this product range (up to $280), ATI X1900XT has better performance (about 15%) than the NVidia 7950GT, and in some regions/stores it has a smaller price. Two main disadvantages: 1)only 256MB; 2)high power consuption.
Only available while stocks last since it has been replaced by the X1950XT.
The NVidia 7950GT is a good card with 512MB (which in theory lasts a little longer or gives a little extra support in very high resolutions), with less heat production and less power consuption. It has a higher price than the ATI X1900XT.
Both are DX9.0c compatible and great cards now.
I believe for gamming or other non-professional use it isn't necessary. Quadro is more interesting if you are a professional that works for example with CAD applications.
Hope this helps.