I should have known 

So the Palincomp one, running down the specs to see what may be needed above the £500 price tag:-
CPU Cooling - is it worth upgrading for £15? I'd think about this, cooling fans are noisy and if it's going in a bedroom a quieter machine is nice
PSU - it's a 650W standard should I go higher? 650 is plenty
Keyboard & Mouse - would upgrade to the USB ones for £10 I'd stick with the ps2 ones. When they break you can buy USB replacements
Card Reader - do we need one and what's it for? Memory cards off cameras, MP3 players etc, useful but available as an external add on for cheaper on Amazon
PCI Slot - do we need one and what's it for? It's not the slot, it's asking if you ant to add a PCI card which gives you more USB slots or a firewire slot. If you have lots of things you plug into USB ports then it's useful, though personally buying an external USB extension is cheaper and more convenient. Or for Firewire do you have a camcorder with firewire connection f not, you don't need it.
Cooling - for £10 is it worth adding two case fans? Wouldn't say so, only needed if you are running a gaming machine
All the other bits seem fine.

