Check your cable isn't broken, there is no reason why AMD cards don't run off HDMI from boot. I have 4 AMD graphics cards and they all run fine off HDMI.
What I have done is A/B swapped them in several sittings. The ATI cards give a display but it is not view-able, like several images on the screen at once. The Nvidea's are totally view-able from boot to desktop. Unless I'm missing something in the ATI setups, but this has happened with two or three different ATI's.
I doubt there's anything wrong in the cables. As a check, I have tried that. The ATI card is a HD5870 from XFX.
Also, the latest update on the ATI card kept going back to default/setup settings on reboot. I had to purge the upgrade and go back to the install CD settings to retain the settings I want. This happened three times, so I'm a little disappointed with ATI now.
Also, what frequency is your screen running at? Try changing it to 60 Hz if it's not at that.
OS Name Microsoft Windows XP Professional
Version 5.1.2600 Service Pack 3 Build 2600
OS Manufacturer Microsoft Corporation
System Name HDTV
System Manufacturer System manufacturer
System Model :ASUS P5KC
System Type X86-based PC
Processor x86 Family 6 Model 15 Stepping 11 GenuineIntel ~2405 Mhz
BIOS Version/Date American Megatrends Inc. 0701, 7/3/2007
SMBIOS Version 2.4
Screen refresh rate is, and always has been 60 Hz. I'm not sure what the above has to do with the simple fact that one works and one does not.