If I were you, I'd do what I'm doing: building a low spec, low energy PC for backups/bittorrent etc.
I'm using a Via PC-1 low power motherboard (uses less energy than a lightbulb!) which set me back around £40.http://linitx.com/viewcategory.php?catid=207
They're a FlexATX form factor - the smallest size, I believe. I panicked at first when I found FlexATX cases incredibly hard to come across, but it's okay, FlexATX motherboards also fit inside MicroATX cases.
The board comes with a processor too, by the way, which makes it very, very cheap. Don't expect to be playing crysis on it though, I plan to install text-only FreeBSD on mine (note: it's not running yet as I haven't got the cash for a power supply or hard-drive), though you might get away with XP or Ubuntu 9.04 and downwards. I'm really not sure though.
Have fun anyway, it's my first time building a PC too.