I see what youre asking...
1)Who is your dialup ISP?
If you are using AOL/or something to that effect, Open the program build 2 different connections ... one for dialup, and one for highspeed. In the future, When you start aol you can pick out of the drop down menu how you want to connect. If you pick Dialup... it will use yoru modem, if you pick highspeed, it will use your NIC. Each configuration is associated by your software to use the specified piece of hardware to connect.
2) You only need 1 NIC.
Why? If youre out and about you have 2 options:
1) you can plug into a router and open Internet Explorer Tools ->Internet Options ->Connections Tab and specify "never dial a connection"
Next, right click "network places" (in classic view on your destop) ->properties... right click "local area connection" select TCP/IP and go to properties. Select Obtail IP address automatically and select obtain DNS automatically. Done. Open IE and you should have internet.
2) If you choose to dialup: open Internet Explorer Tools ->Internet Options ->Connections Tab and select "Always dial a connection and it will dial your default connection.
If youre using AOL, you will have option to dialup or use highspeed from a drop down menu once you launch AOL.
At your home workgroup you said you need one NIC to connect to your home network (true) and it will not access broadband. Something in your workgroup, or configuration on your laptop isnt configured properly.
With that single built in NIC you should be able to plug right into your home workgroup and get broadband, Provided that the rest of your workgroup computers connect via broadband. There is no need to add another NIC. If you do you will be adding to the problem by throwing more hardware and connections into the mix that dont need to be there which will be more confusing to you.
Once you are on your home workgroup, plug into a router and set TCP/IP and DNS to auto detect settings as outlined above. It will obtain IP's from your router and you should be able to open IE and have internet and remain on your workgroup.
In a nutshell, all your workgroup computers plug into a router which goes out to broadband. Each computer in your workgroup has a unique computer name and the same workgroup name. All computers plugged into your router will have internet access. Tell me if your's is different.
What you are trying to do here isnt very difficult, but its just tricky learning how to manipulate your connections. Look here
http://www.wown.com and get a good understanding on how connections/workgroups are setup
1) what is your Dialup ISP?
2) what browser do you use for dialup and for highspeed?
3) when you said you have one NIC plugged into your workgroup and you wanted one more for broadband, i assume you have broadband in your workgroup correct?