Baldy18-
Prior to getting cable at my home, I had a brief stint with ISDN service. Also, until a month ago (T1 installed), I also had an ISDN line here at the office for 2+ years.
Basically at home, I replaced my analog lines with one ISDN against the advice of the telco. Their largest argument against removing analog dialtone completely from the home was in the situation of a power failure. As you know, POTS (Plain Old Telephone Service) lines supply a small amount of power to your telephone equipment. Normally this power is still available when full power from your electric utility is lost and you can still make phone calls in an emergency situation. ISDN lines require that you supply power through the CPE (Customer Premise Equipment) and could leave you without telephone service during an outage. I had at least 2 cellular phones at the time and decided that it wasn't a good enough reason to shell out an addition $20 per month.
Most likely, you will be able to transfer your number to the ISDN line but also remember that the telco will surely charge you the $40+ to reinstall a POTS line after you get rid of the ISDN when broadband becomes available.
I half disagree with CyGoR when he suggests that single channel ISDN access is not worth the upgrade. In an ideal setting 56K and 64K would be similar preformance, however in reality my POTS line would only deliver 20-24 max. I basically doubled my speed and at that time I was thrilled with the boost. Not to mention the call setup time under ISDN is seconds compared to nearly a minute with analog. Similar to your situation, I was able to replace POTS and ISP analog service for single channel ISDN ISP and use the other line for just about the same cost.
With residential service, cost/call should not be an issue, but make sure of the pricing model. Normally you would get a block of calls under that $50/month rate. On the other hand, at the office here we got charged per call per channel setup which under regular circumstances was fine. I "nailed up" the connection to our isp anyhow so our call volume was minimal. But when they had server issues and connection/authentication was a problem, our call cost shot through the roof! It was a crap shoot in that regard.
As far as equipment goes, I am a big fan of external modems/TA's. If there is ever a need to reset the ISDN T/A or to reinitialize the ISDN line, you don't have to reboot your machine. Also, if you are going to replace your analog line with the ISDN, you are not dependent upon your computer system's health to continue to make phone calls.
I have:
[*]2 Netgear XM 128 T/A's
[*]1 Eicon Diva T/A
[*]1 Netgear RT338 ISDN Router
Seeing that I no longer utilize ISDN at home or at the office, I will part with them cheap (read as: really cheap if you are interested). I will part with the T/A's for the cost of shipping and the router for 50$ plus shipping. All devices have 2 POTS ports for hooking up analog equipment (this may answer jliechty's question regarding analog/ISDN call compatibility), each has a serial port for management, and the router also can be configured via telnet.
Hope I shed some light, feel free to post more questions or point out the ones I may have missed. I am reachable via PM and through the email in my profile as well.
Good Luck!
Osmo.