My MX300 was having compatibility problems with my motherboard in Win2K so I got the MX400. I am not at all proud of this card. It had infinite problems with the gameport (fixed by installing the oldest possible drivers), there is sometimes sound distortion in Win2K, enabling Direct3D will screw up most of the sounds in games AND in Win98. A3D 1.0 isn't worth crap, EAX 2.0 is really not true 3d sound at all, and their own canyon API....well nobody uses that.
Keep in mind too that Diamond (which changed name to S3, then changed name to SONICblue) has discontinued the card, and will most likely not provide any more driver updates (think windows2000). I'm in a pretty awkward situation right now: my MX300 does everything better except win2k, and my MX400 does win2k but everything it does drops a notch in quality.
It's really a dreary time for A3D 2.0 right now. It's the best API, but Aureal has just been bought by Creative. Who knows what they'll do.
Vlad