First thing first ,
Mac os X can run multiple network cards at the same time , if you are using for example 2 internet connections you can run them both configuring the cards without using mac os x server .. on the other hand if you want to run a DHCP network on your mac you can use mac os X server to run that DCHP service on your machine and enabling the use of your mac as a routing service machine.
Drivers part of you question , mac os x and mac os x server share the same set of drivers what will work on mac os X client will work on mac os X server your card may bear some specific part of code to fully enable your card's capacities but there are no special differences in between the two.
Side note if you are looking for a great series of ethernet cards you can look here : They are pci-e and work great.
http://www.small-tree.com/
It all depends on the intended use of your second card ,
If it is to provide service to a home network over DHCP then mac os x server is highly recommanded though i suspect there are some utlities out there enabling the feat and that would run on your machine. Mac os X server is a BIG pacakage and will surely get you there but it is like swatting a fly with a boeing 747 ... it will sure kill the fly mind you and without you thinking twice about it either. On the other hand keep in mind Mac os X server's set of services works great on fixed IP networks , if you reside on one of those and have a fixed ip for your machine you can use that machine to host your online stuff host your websites seamlessly run your own blog server using the weblog services serve AFP FTP SMB DNS MAIL Open Directory NFS , host the user partition of your other home machines , setup your security using its built in Firewall and even host the Apple software update service for your other macs.
If this is just to setup two different interfaces that will work independantly then you can press on with mac os X client.
Apple will surely recommend you upgrade to mac os X server keeping in mind you will want at some point expand your capabilities which is a good thing in itself . Mac os X server sports a gateway Setup assistant making the setup very easy.
Another possibility altogether is looking up the following offering :
http://sustworks.com/site/prodipnrxoverview.html
or
http://www.menandmice.com
which provide utilities for setting up both DNS and DHCP services.
If you are a fan of web browser interfaces there is a company named tenon providing a set of utilities derived from X11 named Itools that will run on mac os X and or mac os X server .. the address would be :
http://www.tenon.com
My personnal recommandation :
Full server deployment (the full treatment) : DNS service by men and mice and mac os X server ...
Great stuff all already there : Mac os X server .
Good stuff that runs seamlessly : Tenon Itools.
Small utility that provides just the services you need : Sustainable softwork's ipnetrouterX and their full line of products ...
I hope this answered your questions and helped .
Best regards.
MacPro 3.00 16GB Ram 4*500GB X1900XT Mac OS X (10.4.8) Mac os X server 10.4.8 Universal.