Multi-Speed cards are capable of running the required voltage for their AGP standards.
AGP 4x is from the 2.0 standard and uses 1.5v
AGP 8x is from the 3.0 standard and uses 0.8v
The Radeon 9800XT is 4x/8x compatable, it switches signal voltage by detecting what slot it's in.
The reason your board maker issues that statement is because some transitional cards a LONG time ago claimed to be AGP4x, but used the AGP 2x (1.0) 3.3v signal. Those OLD cards are the ONLY ones that can damage your board.
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>