The Asian hardware site HKEPC compared the ATI Radeon X1800XL with NVIDIA's GeForce 7800 GT in 3DMark05.
Their test system had a AMD Athlon 64 FX-55 processor, Gigabyte nForce 4 SLI motherboard and dual-channel GeIL DDR400 CL 2-2-2-5 memory.
It's still early but it looks like in 3DMark05 ATI's X1800XL won't be able to keep up with the 7800GT. The X1800XL scored 7016 3DMarks while the 7800GT scored 7307 3DMarks.
The full results can be seen here.