Server 2008 is the same as Vista, which made a bunch of changes to the kernel driver model. To be properly certified, you have to do a lot of magic, which includes signing to make sure you don't attempt to sidestep the kernel-level DRM for media(!) Perhaps vendors are waiting until enterprise in general actually upgrade to 2008 on a wide front before porting their product.
XP/2003 64 will also be a problem, because the driver support there is generally spotty. Vista changed the driver model, so any 64-bit port to XP/2003 will be targeting ONLY that particular OS installation, and most systems that are running that don't need new hardware.
I think the biggest problem is your budget. If you want enterprise-class hardware and software support, I'd suggest that about $500 US is the price point. The $100 cards don't really worry about the cases that a high-end enterprise would worry about (and which you seem to worry about).