Unfortunately, yes - BIOS is upgraded to latest (A08) & AGP slot should be compatible. Dell's best is to say just that 'the graphics card is incompatible with the chipset' - so no help there, either.
Next question, I suppose, is whether anyone knows of an AGP dual-link DVI card that does work...
I've recently purchased a new Sapphire Radeon x1600 Pro AGP 256Mb to drive a monitor which requires dual-link DVI. Unfortunately, my ageing Dell Dimension 4550 refuses to boot once the card is installed - a BIOS boot failure (categorised as 'other failure' according to the clever lights on the...