Here's what I would try next:
If you have a "VGA Mode" option in your Windows 2000 Boot Menu, try that one. It loads with standard VGA drivers vs. your videocard's normal drivers. Video drivers can cause Stop Errors on bootup. If you don't have a "VGA Mode" option on your Windows 2000 Boot Menu at bootup, do this from Win98:
Go to the c:\ folder. Right-click on "boot.ini" and choose "Properties". Remove the checkmark for "Read-Only". Now, close the properties and doubleclick on the boot.ini file to edit it. Find your entry for Windows 2000 Professional. It should end with the switch "/fastdetect".
You want to replace "/FASTDETECT" with "/BASEVIDEO /SOS" (all of this without quotes). "/BASEVIDEO /SOS" will load VGA drivers and ALL other normal drivers, which is different from Safe Mode which only loads limited drivers. "SOS" will list the names of the drivers as they load, so should there be a problem with a particular driver, it should be the last one on the list that's loading. Now, it could also be the next driver on the list at startup that's causing the problem, and if that's the case, it won't show on the list of loading drivers. However, you can find the order of driver loading by booting up in SAFE MODE in Win2K and going to Regedit.exe, then going to the following key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\
Control\ServiceGroupOrder
Make sure there's a space between the "O" in /BASEVIDEO and the "/" that preceeds "SOS".