You might want to consider a power supply upgrade too. Cooler master used to only make crappy power supplys. Now they make both decent ones (wouldn’t be my first choice but they are suposed to be decent) and crappy ones (with heavilly exagerated ratings). If you buy just the power supply they are easy to tell apart as the crappy ones are less than half the price of the good ones for similarlly rated power supplys. I’m guessing the included one has a good chance of being a crappy one but you can see what you get when you get the case. Going off an average system (adding things like cpu fan and two small low power case fans, sound card, 1 hard drive and the above equipment), but no usb/firewire devices, I get 278 watts.
I’m not sure I would trust that to a crappy 350 watt power supply (that probably doesn’t put out near the claimed 350 watts).
You can get a good 350 watt pretty cheap (I know that you are in the uk, this is just for example)
Though, the cpu and vid card (normal x800, not taking into acount its an all in one) take about 150 watts alone (12.5 amps), so you might want to look for a 350 watt with a bit higher 12v rails or perhaps a bit larger power supply.
Of course I’m a bit of an alarmist when it comes to power supplys, but for good reason. You can always run prime95 and test the voltage on the original power supply with a multimeter (or software though thats not as accurate) and see if it runs it stable.