I find that there is not a massive difference between most PCI modems.
My mate has this modem and I put one of these in the system I built for my brothers, It's a good modem, you may as well go for it.
If your old motorolla is a 28/33K ISA then it will give you a nice performence boost.
If it's an ISA 56K then the performence increase won't be too noticble unless you were having big problems with the old one.
Diamond do claim that it's "built up from the ground rather than a re-designed 33K Modem" and the software is adequate, a good a choice as any I'd say.