You might try several things - my apologies in advance if you've already done this stuff.....
1) Try uninstalling darkstone and reinstalling it. In many cases, when games initially install they check the video card and set themselves up to use it's features (cuts down on install space requirements). If you change to a new type of video card, you should re-install any 3d-accellerated games you want to play on it (that were installed using the older video card).
2) Go out to diamond's web page and download their latest patches, then re-install direct x from Microsoft's web page. (MooSquish doesn't readily tell you this, but each time you update sound, video, and/or multimedia drivers, you should re-install direct x afterward).
3) Make sure you are running in AGP mode. In your bios, set the "Initialize VGA/Video First" option to AGP instead of PCI (default). Note that AGP uses USB, so make sure that USB is enabled and is properly installed. Also make sure that both the VGA card and USB options are assigned IRQ's. Then go out to your motherboard manufacturer's web page (wim's bios page has bios sniffing shareware that works well) and download, and install, the VGA or AGP inf file (for the VIA based motherboards that I distribute you would need to properly install the 4-in-1 driver from VIA's website). Note that most MB manufacturers are in Taiwan, so the web address is
www.pcchips.com.tw (PC Chips TW website).
MTCW,