|You should look up the benefits on a Windows 2008 domain vs. a Windows 2003 domain to answer your questions. A simple search on Bing will get you those results. |
As far as the graphics card on a domain controller, it isn't an issue. The domain controller should need any fancy card. In fact, my Windows 2008 domain controllers run the Core version of the OS, no graphic interface at all.
You should have more than one domain controller, especially if you are running Exchange. If you lose that domain controller, even for awhile, Exchange stops working. If you lose the domain controller for good, you pretty much lose your entire domain unless you can restore from backup - not a fun procedure and not something I'd want to be responsible for.
Personally, I would add another domain controller. If you can get a 64 bit server, I would add an R2 domain controller to your existing domain. You will have to prep the domain accordingly, forest prep, domain prep, etc., but I would not have a domain running on only one DC, especially one running Exchange. If you can't get a 64 bit machine, I'd add a 2008 DC.
Windows 2008 will not give you all the benefits of 2008 R2 but either will running 2008 R2 with a domain controller running 2003 in the same domain.