I think as supplied, this PC uses the on-board graphics (built into to the m/b). As this PC is 2nd hand, it may already contain a graphics card.
To fins about your system easily without opening the case you could go to www.piriform.com
and download a program "speccy" (free). Run this and it will tell you the details of your system including the graphics card type and detail of the memory. The simple memory can be obtained from Windows > My computer > (right click on) properties if I remember correctly).
Next go here
You will find the minimum system requirements for playing WoW. You may find that to upgrade not only the graphics card but also the RAM. If you need and decide to upgrade the graphics card be sure to shoose one that is compatible with your computer with your computer/motherboard. I suspect (http://www.pcadvisor.co.uk/forums/1/tech-helproom/69141/packard-bell-imedia-5064-motherboard/
) it may be very similar to this one
but I don't know for certain. This page contains all the details for that motherboard. Note that speccy will give you some detail about your motherboard and memory types required as well.
Depending on which graphics card you decide (this may be helpful http://www.playtool.com/pages/vidtable/table.html
) on you may need to upgrade your power supply (PSU). It's very easy to do this; more difficult is to decide on which PSU to choose. You may need to some research (as with the graphics cards and memory). There is some good second hand stuff - PSUs graphics cards memory often available through ebay but caveat emptor - read listings very thoroughly and carefully, don't pay too much (set your limit before you look).
syetem requirements for WoT are here http://worldoftanks.com/en/content/guide/general/system-requirements/