What kind of motherboard do you have at the moment G-man? If you have one that takes AGP graphics cards, then you would have to buy a new mobo (and maybe a new power supply) to get a PCI-E graphics card. If you mean just a PCI graphics card, AGP is better. If a mobo can accept an AGP graphics card, it should take a PCI one. The main difference at the moment between the three is communication speed and bandwidth. If you have an AGP mobo, I personally would recommend you stick with it, the price of the upgrade alone should put you off...
www.komplett.co.uk have always been ok with me, as have ebuyer and...ebay. Although thats not quite the same, I know.