The 7800gs AGP is severely capped back compared tot he GT. However, I'd think it would still crush a 5900Ultra.
7800GS:
375Mhz core clock
1200Mhz memory clock
256bit memory interface
256MB memory
16 pixel pipelines
5900 Ultra:
400Mhz core clock
850 Mhz memory clock
256bit memory interface
128MB memory
8 pixel pipelines
See. Even though the 5900Ultra has 25 more Mhz core clock it dont mean s*** for the fact that the gs has twice the pixel pipelines. Even if the gs is an anemic version of the gt, it would pound the ultra into the ground in almost anything.