only 'real' advantages of v92 that im aware of is faster upstream (i.e, uploading) speeds and theyre meant to be quicker to dial in the connection. also one big advantage for those with one phone line (lets face it, nobody using 56k would have two, theyd get broadband) as they can detect an incoming call and put internet on hold and the call comes through, soon as you hangup it automatically resumes what it was doing. Above is "so they say anyway".
however no idea how many ISP's will support it. id be surprised if its not most as theyve always taken up new techs in past, although this first time when its not been a headline (i.e. DL speed) improvement. bear in mind all v92 modems will happily work as v90 so no reason at all not to go for the v92 version is teh cost diff is negligible. personally my modem i s'v92 ready' and im swapping it for anothe rv92 ready mostly because i want to be able to receive calls and also anything to minimise MP gaming ping is great for me.
i would also get PCI over ISA, not for any speed diff's (if theres any at all) but because PCI will be on new boards for a long time, few current boards have ISA already, and if u need to change the mobo for whatever reason in future you would be forced into buyign a new modem.