I've the same problem with two Digi Acceleport RAS 8. I've the latest drivers (5.0.314.0) and I'm using Windows 2003 R2 SP2.
There's something unusual with this configuration. We have 4 servers with 2 cards each, sometimes it hangs the OS and sometimes it doesn't answer the fax calls. I can only recieve about 250 faxes. With only one card the system works like a charm recieving at least 2000 faxes.
I'm using FaxNow 5.02 build 799.net and HP dc7600 CMT.