I'd recommend Overclockers.co.uk if your looking for a pre built box.
For the same price as that PC world one you could get THIS
It has a better CPU and a better graphics card, and a less obtrusice case. That Advent one look TERRIBLE!
Only downside is you have to pay extra for Windowz and it has a smaller HD but they are cheap as chips now.
I'll even send you one of my spares rather than you spend money at PCWorld lol.
The mobo is an AM3 so you can upgrade to the X6 chips if ever you get a bit of wedge in the future plus I think it may have the tech that was in PCGamer this month about unlocking the extra core in an X3 chip (but you'd have to research that more)
It has 1 x PCI Express x16 slot 2.0 (taken by the gfx obviously) 1 x PCI Express (does anyone even use these atm?) and 2x PCI slots for soundcards etc. so even though its a smaller form factor mobo there is room there for expansion.
If thats still stretching the budget try this other AMD SYSTEM
or this Intel Core i3 SYSTEM
Note: The intel system will NEED a gfx card if you were going to go down that route. The onboard, although better than they used to be, is not made for gaming in the slightest.
Sorry I'm getting techie, comes from years of building them for friends/family and wanting them to get the best for the money (usually involving me with scuffed fringers off chips,cards and screws :P)
Nothing is more satisfying than building your own and it booting for the first time.
They're not made of fairy dust and spiders webs, you just need common sense and a bit of time.
If youve got 5 hours it takes 5 minutes and if youve got 5 minutes it takes 5 hours (usually cos you borked it cos you rushed lol)