My apologies. "Wasted" was my own thought from the comments, I shouldn't have implied it was yours as well. Again, sorry about that.
The drivers I researched (and downloaded - I'll test them out eventually,) are all for reading UDF disks in native Windows 9x, not for writing them. The link I gave actually states its for "reading only" of disks written with the software you have to buy - and that these drivers come with that writing software so you won't need them. So as long as the DVD drive itself is capable, then those formats can also be read in native Windows 9x. That's all I was trying to say really, that UDF reading is fairly easy in Windows 9x.
None of the drivers I downloaded, however, specify a UDF version limit - so I don't know how high their support goes beyond 1.50 (the RW version.) I have no idea which (if any) of them supports 2.0x (2.01 was a bug fix really,) or 2.50+ (mainly for Blue-Ray & HD.) According to what I've read, drivers for versions 2.50 are available, but I cannot confirm it. I don't know of any non-SATA Blue-Ray drives though (but I haven't really looked since I bought mine.) So, I'd be willing to say that I wouldn't advise trying to get a Blue-Ray running in an older (Pre Windows XP,) system.
Basically, the original question was, if a DVD would work in an older system of if they "had" to use a CD. They didn't state the age of the system though. I'd be willing to go with a general rule of thumb.... If it has PCI slots, a DVD drive should be fine. If all you have is ISA, a DVD drive is risky, and it might be safer (assuming you haven't actually bought either yet,) to just get a CD. As someone said earlier, if you already have the DVD drive and are trying to find out if you need to buy a CD, then I'd say don't bother just yet. Install the DVD, and if it doesn't work, go looking for a CD.