Okay then, so your server must service as follows:
1. A gateway to the Internet (Proxy)
2. An email server (Pop and SMTP)
3. File server
4. Printer server
You will likely not require a large HDD for file storage, however, one has to take in to account caching of Internet documents by the proxy and also space for temporarily storing emails until they are picked up by one of your internal mail clients; or sent out to the Internet via your ISP.
You should therefore be able to cope with an 80 Gb HDD however you might as well spend a tiny bit more and get a 120 Gb HDD. Whichever size you get, I can not recommend the Western Digital Caviar range enough, mainly because of the 8 Mb cache (they go like lightening), the fact that WD have been the most reliable of all HDD manufacturers we have used in clients' machines, and that they are really quiet (you also get an extended 3 year warranty). I insist on using the Caviar range of Western Digitals if RAID is not being used.
One valuable piece of advice: make sure that there is a HDD fan on any HDD that runs at 7200 RPM or higher. Otherwise there can be a serious risk of the HDD over heating and damaging itself.
On the processor front, you do not need the most powerful CPU on the market in terms of 3 GHz etc. However one might as well use something such as the AMD AthlonXP PR2600+ which runs at just over 2 GHz and has a 333 FSB which aids in performance.
Memory is a more interesting area to talk about because, really, it would depend on the OS to a certain point, naturally. If you are planning on using Microsoft software for the OS then I would recommend using 1024 MB PC3200 (PC2700 at the least). Windows runs more smoothly with increased memory alone and not more processor power. However, if you are planning on using a *nix version of OS such as Linux, 512 MB should be quite capable, with an added extra of another 512 MB making things run even more efficiently if you desire.
On the motherboard front, any in something like the MSI KT4 Ultra series should do since they have onboard LAN, take at least two sticks of PC3200 (running at 400 btw), copes with the 333 FSB on the CPU, has various USB and Firewire (1394) ports (perhaps for backup purposes), and with lots of expansion space for PCI and AGP cards. Audio will come as standard on a board such as above but you may not need this for a server.
If you buy a motherboard (such as above) then you will also need to buy a case. Any standard full ATX case should be fine and make sure you get at least a 350 W PSU. It may also be worth investing in some internal fans for the case in order to make sure it is well ventilated inside. If you don't get a motherboard, you could try something like a compact but well ventilated Shuttle XPC which are, contrary to many critics belief, quite capable of acting as servers, as long as you have no major plans for internal expansion (there is plenty of room for external expansion with USB and Firewire on XPCs).
I hope this is of help. This should be the basis of a powerful server for your workgroup. Will you be requiring advice on proxy and mail server applications/daemons? If you have any queries about this or want to discuss the spec further then just post away.