Some Advice
PCI graphics cards are cheaper than their AGP and PCI Express counterparts. However, they are also slower than the other two metioned. I personally like nVidia cards because they have a unified driver system. That means one driver will run any nVidia card; so you don't have to worry about finding the right driver for your card. To answer your question about which one is better, here's a simple answer: The higher the model number, the faster it is. Some companies also make O/C or OC versions. This means that they're overclocked and will run faster than what is on the box, although, they can get pretty hot. Some cards have a TV tuner in them and you can record TV just like Tivo. So its up to you and how much you want to spend. The only recomendation I'll give you is to buy a card with 128Mb of RAM or more.
Happy hunting! :grinthumb