I have asus p5wd2 mb and asus eax1600xt video card on pentium PC (OS: Win XP).
My computer doesn't start, so I get a beep message like I get without a video installed.
I have installed an powercolor x800 gto and the same - nothing
I have instaled an gigabyte 6600 and it woks perfect
I have changed the monitor with a 17 inch dell and it worked with both ati cards.
All components are brand new (monitors are not)
In conclusion all video cards are good and my 21 dell but they does not work together. Any sugestions?

it could be a bad connection in a cable somewhere, or possibly in a monitor. Some motherboards/video cards are more sensitive to shorts and miss communication than others. This could just be one of the freak combinations with the used hardware.
The other thing you could try, as i've seen this work, use the monitor/video card combo which works and uninstall all drivers for all video cards and monitors, including software which may hold values relating the 2.
Then shutdown your comp (without detecting new hardware) and install the desired parts and boot. Without any drivers or software possibly telling the boot.ini file and bios what to expect, it might just work

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I have had some weird issues with ATIs in the past. I had a 9600xt that if it ever cooled off it would give me garbled text for 20 minutes until it got warmed up. So if the pc was shutdown you would have to wait like 20 minutes before you could see the display.

As mentioned check connectors and everything else.