I've got a server style PC which will be re-employed as a server once I build myself another PC.

The specs are:

Dual Xeons @2.4Ghz
4 GB RAM
About 1 1/2 TB HDD, (though I keep adding and taking drives out so god knows how much at the moment)
Radeon 9800 Pro Graphics.
Dual 22" TFT Monitors

The only problem with it is I can;t get drivers for anything after XP, (Vista/7/Server 2008)
It's currently running XP, but I'll install Windows Server 2003... when I build my next PC.

I am planning to build a storage server/media pc/seedbox. I am waiting for a new case, when I get it I'll use old Coolermaster Wavemaster to house the server with my HDDs. In my current rig I will have only SSD drive so it should be near-silent PC. Right now HDDs are the loudest part of my rig and I can't stand them...

Server will run Vista or XP. I would go with Linux if it could provide GPU acceleration for HD movies, which it can't. I plan to use spare C2D E6750 with 2GB ram but I will switch to next gen IONs when they come out (end of 2009?)...

i might be getting one although im considering a nas external hardrive

If it is just for data storage, not running a full on mail server or web server, then I'd go for a NAS these days. They are generally cheaper and they save a heck of a lot of electricity, in comparison to a "real" server.

I've seen a load which also act as a web server.

Not sure about mail, but there are most likely options out there.

I can't get over how powerful most of these machines that people are running as home servers are! Beefy dual core systems to handle what, 50 emails a day and a small amount of web/file transfer traffic?

The box I'm currently looking at is a WD MyBook. It has a 200MHz processor and 32MB ram

So far, I'm not very impressed. It takes ages to serve up a web page, the file transfer speed is very slow, it's incredibly noisy and it uses a lot more power than I expected. I've not even installed half the stuff I want on there yet and it's already struggling.

I may look around at the alternatives, or just go back to using the Athlon

E6400 at stock
2 gigs of 533mhz ram
160gb & 300gb HDDs

We use it as a file server for pics, music etc...

Actually i've got to say...

I've run a game off my server >.<

Was sat in the front room with my EEEPC and wondered to myself... "I wonder if I could stream Civ 4...?"

Since all my games are backed up as DVD images on the server and I had the CD Key on my bookshelf downstairs already (All games in a games folder with relevant CD keys for when I travel to parents house) I installed it onto the server from the front room and then proceeded to play it via the WHS console.

Not sure if it's standard, but I used remote desktop and played Civ 4.

=)

Considering all my other machines are quad cores @ 4GHz with 8GB+ of RAM it's very conservative.
I stream music & video all over the house so didn't want anything to be too slow.

If that's all you're doing then it's a bit silly, however when you start virtualising operating systems for compatability etc., you soon need the extra oomph, I certainly wish I could upgrade beyond 4GB of RAM, and swapping to a CPU with hardware virtualisation would be really handy too as it would allow me to virtualise Linux (Red Hat and Suse IIRC) straight on top of the Solaris kernel without the need to use VirtualBox.
Also, when the Sun Ray/app server gets installed on it I'll have at least two, if not more users on there at a time, so the extra processing power will be good for Firefox alone...