I will explain it clearer, in choosing a graphics card you need to ensure you have a reliable power supply which will ensure that when you buy a new component like a graphics card that it doesn't cause damage to what you have bought, with low quality, cheap power supplies this can go as far as destroying the whole system.
Second, although this is in no particular order, you need to ensure that the power supply is powerful enough to power the new card, a power supply takes the input (240v in this case) and then converts it then the output is a lower voltage. I does not feed 240v into your computer.
On the computer if you take it apart on the side on the power supply unit it should say the model number you can either gives us that or the wattage, for example mine is 650watt, and if possible the amps on the 12v (it should say it in small writing on like a chart)
Buy the sounds of it you have a advent pc that has the same psu as when you bought it, basically the stock psu, is that right?
Finally what is your budget? I mean figures not currency like x amount of £s, there are literally thousands of graphics cards to choose from ranging from cheap to expensive.
What do you intend to do? you say you are not a gamer, do you play games at all? will you be watching HD videos and such? you will end up with a card with at least 512Mb btw no matter how much you spend as I dont think many places still make cards with that little memory, atm having 1GB is just about becoming standard on graphics cards even the lowest end cards.
It is very easy to install, you should have no porblems there when you get the card just open up the case and put it in the correct slot (pci express) then close it all up and start the computer, it will detect the new hardware automatically and then you should download the drivers from the correct website, usually nvidia or ATI/AMD website.