The other thing with graphics is that to make it worth while you are going to have to spend more than �50 (anything less will proably not be much better than you have now).
And it is probably not worth spending that on a PC 4 years old.
Also, as someone has said, the slots for the graphics cards have changed in the last few years.
The old slot was called AGP (replaced by PCI) and if your PC has an AGP slot you are going to find it hard to get a decent card as most people are making PCI cards now.
You can buy a brand new "system unit" for around �200 (without monitor, keyboard, mouse, speakers etc) and you are far better puttng the money towards that.
Finally, Packard Bell is a poor PC World brand, try to find a local PC builder who will put a PC together for you, then it will be easier to upgrade it at a later date.