http://service.futuremark.com/compare?3dm06=338395
FYI:
I left it at what I believe is stock x1800XT speeds of 600/700. I was under the impression that XT's run at 600/750 but all the default x1800xt bioses that I found had 700 for memory and comparing to Orb it appears that 600GPU/700MEM is correct. Either case I plan to leave it at this level for a week to confirm stability but after an hour of ATI Tool artifact tester, some BF2, and 3dmark it appears I'm good to go.
Also I found the bios listed in this thread, as well as the ones on techpowerup and mvktech, appear to be black listed from the CC 6.5. An example is if you try to uninstall the video driver via add/remove programs, restart, and try to install the driver it will list unknown inf. You need to install the driver manually or revert back to the original bios.
I decided to go back to the original X1800XL bios to resolve this issue with the driver installation after cycling through 4 online X1800XT bioses and experiencing the same issue. Since the only thing that was different between the default X1800XL and the X1800XT bioser were with clocks and voltages I used Rabit.
I performed the following on the original bios with RaBIT:
1) I modded it to 1.32 volts on the GPU (default for the x1800xt). In RaBIT you need to modify in 10% increments and save/close/open to up it to 1.32.
2) Upped the mhz to 600 on the GPU
3) Modified the description so that CC would indicate the core mhz listings
4) I left the memory voltage the same since ATI tool was already listing 2.0 volts to the memory.
5) Upped the memory to 700mhz.
After those changes I'm good to go and the driver issue was resolved.
I could be wrong on CC blacklisting the online bioses but this is what in my opinion is happening. If you could get a MSI X1800XT, or any non-blacklisted bios, I'm possitive it would save all the steps above.
On my browsing around I did confirm that this model is using the new X1800XT circuit board:
http://www.xbitlabs.com/news/video/display/20051229212113.html
Pn# on the board lists the matching 109-A52031-20 supporting 700MHz/1600MHz.
So that is my report. It will be interesting to see what others post but so far so good.
**Edit**
As far as my comment regarding blacklisting I believe it's more of ATI excluding the pn# in the inf file.
It could be as simple as modifying the inf by hand to include this flashed model. When I get some more time I will investigate further.