The recommendations you will receive will vary depending on the expected use of the server.
It will have 25 thin clients...
--How will they connect to the main server? VNC? x11? ssh? (or a combination, perhaps?)
--Along the same lines as above, will it be a text-based environment for your users, or a graphical one? Not running x.org can save a lot of RAM...
--What is the main purpose of the server? Is it providing basic email and file sharing, or will it also be hosting a database etc. ?
Hopefully we can provide some decent ideas for you once we know a little bit more about the intended use.
Generally speaking, however, I recommend at least a single dual-core processor with a clock frequency of 2GHz or above (Intel or AMD? Take your pick--there's a flamewar discussion right there, heh), 2GB RAM (ECC optional), and at least some form of storage redundancy--either hardware- or software-based RAID.
Network-wise, I'd go for a gigabit ethernet (GbE) NIC (I lean toward Intel-branded adapters in linux, myself) and GbE NIC's and switches for the clients--if they're onsite. If they're offsite 10/100 switches would do, really...
Hope this helps.