Firstly I currently have in my PC a intergrated 64MB shared graphics card, I take it this means its a AGP and PCI card rolled into one.
No. An AGP card fits in the AGP slot. A PCI card fits in a PCI slot. Integrated means your video is a part of your motherboard and not a card at all.
I had a failed attempt at changing it today, when I took off the PC cover I notice that the shared card was in the white slot (the PCI slot I belevie) with nothing in the brown slot which I understand is for the AGP card so I placed it in there but all I got was a blank screen so I reversed the operation now I am back where I started
If your computer works at all after a stunt like that, thank your lucky stars.
You cannot just randomly move cards around. Even an AGP card with the wrong voltage can destroy a motherbord if it's put into an AGP slot.
If you want to upgrade your video card, the first step is to find out what kind of motherboard you have, then you look for a supported card. If you have an AGP slot, you have to know which type of slot it is, whether 2x 4x or 8x. Only then can you find a proper card to fit it.
When you removed the card that you put into the other slot, did you first have to disconnect your monitor cable from it? If you did, then chances are you have a PCI video card already. If you have been using the onboard plug for your video, chances are you have to first disable onboard video before you use an AGP slot. This is all covered in the motherboard manual. Look before you leap.