I want to upgrade the graphics on my Packard Bell Imedia S3840 where can I find the information?

Yes, you can.
RAM, HDD, Graphics card etc.

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
http://www.wowwiki.com/System_requirements 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
http://www.gigabyte.com/products/product-page.aspx?pid=1335#ov 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/

21.December.2014

I currently have installed is Ati Radeon HD2400 AGP 256MB graphics card and im looking to upgrade it to this Sapphire Radeon HD3650 512MB AGP Graphics Card will it risk my computer to overheat, or is my computer not good enough to run such graphics card

would upgrade to better board first jim

I would suggest going to the Packard Bell Support web site then select your country followed by selecting your model of computer which will then give you access to any available documentation to help you with upgrading memory etc.

Click on this Packard Bell Support Download link,

http://www.packardbell.co.uk/pb/en/GB/content/download

Click on - Link to the old page - to the right.

Click on DESKTOP
Click on iMedia (2004-2005)
Click on Driver
Click on Xp

Click on the appropriate graphics driver file, ATI_R480_Chipset.exe,
OR,
Radeon _xpress200_811.exe

Download the Execute file (.exe), and install it.

(I would also download, and install the ATI_Catalyst_8630.exe
It's the 'control panel')

Regards,
joecoolvette

http://www.google.com/search?q=sound+card+for+packard+bell+imedia+5070+&rls=com.microsoft:en-us:IE-ContextMenu&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GWYE_en

Changing the hard disk will not give you any noticeable performance gain, only some extra space. Just get the biggest you can find.
On the other hand your system could definitely be improved by a new video card. The best one, even a little overkill, for your system is something like 
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102851

You can upgrade the memory to maximium of 2gb of ram 1gb stick of ram in each slot. The memory you will need DDR PC 3200 184 pin.

You can also upgrade the hard drive for more space or buy an external drive to store files.