SUS is pretty nice, I am doing it for the college I work at.
Here's the requirements according to microsoft:
Windows 2000 Server
700MHZ CPU
6gb hard drive
512mb ram
I'm running it on the following:
Windows 2000 Server
450mhz CPU
8gb hard drive
256mb ram
My server works fine, but the whole campus isn't setup yet, only about 30 or so clients. MS says that the 700mhz server can handle up to 15,000 clients. I figure mine can at least handle half that, which is more than we have for clients anyways...
Also, you need to be aware that your clients will need to be configured via the registry, unless you have Active Directory setup. The registry settings are pretty simple, maybe 10 total. Read the docs from MS for details.
Since you have to use Automatic Update to get the updates, you can't manually get them anymore unless you go to windowsupdate.microsoft.com. So you have to monitor your server, approve any new updates from MS, then wait for the clients to get them. It can take up to a day or two for all your clients to update from your server.
I hope this helps, feel free to PM me with any questions.
-Josh