Question : Problem: Need Hardware Reccomendations for Domain Controller

We need to purchase a server to be a Domain Controller. It will run Win Server 2003 r2. It will be supporting about 75-100 users in Active directory. I would like at least 1 recommendation to be a Dell server as all of out computers are Dells so far.  

Answer : Problem: Need Hardware Reccomendations for Domain Controller

Any Dell (or any other computer for that matter) will be fine if all it's doing is acting as a domain controller.  Domain Controllers, especially for environments that size, are not at all heavily used.  Even an 8 year old system could handle the job with no noticeable delays for the users.

That said, there are a few things to keep in mind:
1.  512 MB of RAM (frankly, I'd go with 1 GB because RAM is cheap and I'm not even sure you can get less than 1 GB from Dell right now.
2.  RAID 1 Mirror.  Small hard drives are fine, but make sure you implement RAID 1 mirroring.
3.  ANY CPU.  Dual Core is way overkill, but I'd still probably get a low end dual core.  Quad Core is just a waste of money for this function right now.