hardware requirements domain controller

Oh, and I almost forgot Vmware GSX Server 2.5.

Anyway, for 1000 users, I think you can get away with a Dual Intel Xeon CPU at 2.4 GHz, no less than 2 GB RAM.  If you just want to do authentication, then you can save on the storage requirments, otherwise go with a SCSI RAID 3 disks each at 74.3 GB for a total of 140 GB (you may want to get more for 1000 users?? or use disk quotas).

If this is going to be your only server, I highly suggest AVOIDING the use of MSSQL server, MS Exchange, IIS, Internet Accelaration and Authetication Server, Small Business Server.

Just to give you an example, I have about 60 users on a similiar configuration.  I serves as a directory, file, DHCP, DNS, HTTP (local Intranet), database,  RIS, and group policy server.  

IBM eServer
Dual Intel Xeon CPU at 2.4 GHz (2 physical, 2 virtual)
2 GB RAM
SCSI RAID, 3 disk, total 140 GB
40/80 GB DLT Tape drive
Gbit Ethernet

MS Windows 2000 Advanced Server
Microsoft Active Directory
Apache2/PHP4
MySQL 4.0 for Windows
Microsoft DNS
Microsoft DHCP
Roaming User Profiles
RIS
File Server
..and some other things I can't remember at the moment

99% of the Workstation are Win2K clients - you get more out of your hardware with win2k.
All use Mozilla Thunderbird for Email, Firefox for Internet (IE 5.01 is avialble for those problematic sites)
Microsoft Office 2000 (the best!)
Symantec AntiVirus 8.0 (managed install on seprate server)
Microsoft Software Update Services (on a seprate server becuase of IIS requirment)
Jabber Instant Messaging (On a diffrent server, FreeBSD 5.2.1)
RIS allows me to install Windows 2000 over the network, and GP's save me time by having settings and software Installed, updated, removed automaticly
All user accounts use Roaming user profiles, allowing anyone to log in anywhere and have access to thier documents, settings, and email.
TightVNC on all workstations and servers for remote assistance and management (http://www.tightvnc.org).

For Email I use Mercury/32 (www.pmail.com) and eGroupware (http://egroupware.sf.net), while it does NOT integrate with AD, it has many benifits over Exchange.  I currently run it on a seprate server, but you can run it on the same server with everything else with *somewhat* of a preformance impact.  As with the Email, you can also combine your AV system on the same server, but I would HIGHLY recomend getting at least 1 or 2 more servers, one for AV and Software Updates, and the other for just Email.  I would NOT install the SUS server on the primary one (becuase of IIS).  Or, If your really savy and only want 1 server, install VMware GSX Server 3.1, and make 4 virual servers, one for AV, one for SUS, one For Email, and one for Jabber IM.  Beware! this will significatly use up system reasources, and you may need faster hardware, LOTS more memory, and tons of Disk Space.

My users are very satisfied (except for the Mozilla Firefox, our parent company develops many IE-only websites), Mozilla Thunderbird is definetly one of the better email clients I've worked with, Mercury/32 is (in my opinion) the best email server you can use, and with Apache2/PHP4/MySQL in the mix, I've got one of the most stable and secure web servers you can run.

I know I can't compare 60 users to your 1000, but you'd be surprised how scalable you can make a server by choosing the right software.  On average, this server uses no more than 3-4 % of it's CPU during the entire day. Keep in mind I have some major stuff running on seprate servers as mentioned above.  To run it all on 1 server, reasource usage can increase significatly especialy during the morning rush hour.

Hope this Helps!