The GeForce 7900 GT and GeForce 7800 GTX cards produce coinciding results under normal conditions (also at 4x FSAA). The difference is 1-2fps while the average frame rates are 50-80fps.
The GeForce 7900 GT SLI tandem does much better in the 8x SLI AA mode, especially in high resolutions. The new card is sometimes 20% and more ahead of the older one and is competitive against the Radeon X1800 XT CrossFire despite the latter’s higher frequencies and twice the amount of graphics memory. You should bear in mind the overall orientation of the Doom 3 engine, which uses OpenGL and stencil shadows, towards Nvidia’s GeForce 7 architecture.
The gap between the new and older cards is diminishing in the 16x SLI AA mode as the display resolution grows. The memory load is too high then and the 60 (120) MHz frequency bonus doesn’t matter much. The Radeon X1800 XT CrossFire is in the lead, but only due to its less resource-consuming method of doing full-screen antialiasing which only simulates super-sampling. The price of this tandem is much higher, too.