are you sure you have an 8x mobo? If it's not, then it wouldn't give you a 8x setting in the bios.
As for my own mobo, it is 8x compatable, and gives me the option of using 8x in the bios, even though my card is only 4x
the fsb setting should not have any affect on whether you can get 8x or not
if your mobo bios will only go to 4x, then that is why you only see 4x in WinXP
edit:
I looked up your mobo, and it goes say it's 8x..so now my queston is: Is your videa card truely 8x?
As for my own mobo, it is 8x compatable, and gives me the option of using 8x in the bios, even though my card is only 4x
the fsb setting should not have any affect on whether you can get 8x or not
if your mobo bios will only go to 4x, then that is why you only see 4x in WinXP
edit:
I looked up your mobo, and it goes say it's 8x..so now my queston is: Is your videa card truely 8x?