Well, my "media centre" (actually only audio running a Squeezebox server and a few other odds and sods) is currently as follows:
Intel D201GLY2 mini-ITX m/b (60-odd quid including CPU)
1GB RAM
WD 10EADS 1TB Green HDD
Some cheapo case.
Not as impressed as I was with the previous VIA EPIA mini-itx setup which had half the power consumption and would stay up for years at a time. This one has more clout but barely adequate cooling and tends to throw its' toys in this weather.

Wondering if an Atom m/b would be better, or even going back to the EPIA. Or bodge a better heatsink onto the chipset.
Not sure about a desktop. Haven't bought any PC hardware (bar the netbook I'm currently typing on) for many years and kind-of got by with hand-me-downs as my speed requirements have been modest.
Now getting into processing digital photos and a few other new requirements and everything's got a bit pedestrian.

So.. desktop shopping list..
I tend to buy slightly below the silly priced end of the market but something that will be grunty enough to last a while. (current desktop must be nearly 10 years old

)
Thinking about an LGA 1156 socket M/B and maybe an i5 750 CPU. 4GB RAM. Not really fussed about graphics as I don't do games but a dual-head display would be nice. 1TB hard drive. Lob Ubuntu on it and it'll do me nicely.

That's for my needs, though. For just surfing the web and e-mail you can get away with something modest.
Here's something from Skruntie's link:
http://www.aria.co.uk/Systems/Home+Range/Atom/Gladiator+Proteus+Atom+Green+PC+-+No+Operating+System+?productId=40097. PC for 120 quid. More than adequate as an internet workstation. Might groan a bit when it comes to watching video but if that's only the odd youtube clip, so what?
On the other hand, if, unlike me, you want gaming capability, the graphics side needs more consideration, obviously.
Kevin