Not quite true, if the motherboard is old enough (2002 or earlier) it will run into the 137GB hard drive limit because it lacks 48-bit LBA addressing. Also, the OS must support it (WinXP SP1 and above if I remember right.)
Just want to make that clear in case someone browsing the forums runs across this thread.
Your D946GXIS supports 48-bit LBA so you should be able to use a 1TB hard drive (assuming your OS supports it).
^Correct. The 137 gig limit is Windows. Windows XP SP1 and later will support drives over 137gig. Before XP SP1 (That is Service Pack 1 by the way) you could not use a drive more than 137 gig.
But 48bit LBA addressing is also a problem for the motherboard. But that goes back to even before 2002, I am not sure of the exact date, but I believe this something left back in the late 90's. Surely you have a more modern board than that!