PCI-E and AGP relate to the interface on the motherboard that the card fits into.
AGP is now a dated tech that you probably won't see around anymore.
PCI-E is what you'll be looking at these days on almost every motherboard.
The A and N after the interface type relate to whether it's ATi or NVidia Technology in the card.
So overall, you're looking at PCI-E cards for almost every motherboard I know these days. It's then down to what you want from your pc to whether you go Nividia or ATi. For example, not all boards are certified for Crossfire or SLi, so say a board was only certified for SLi you wouldn't be able to run 2 ATi card in it only NVidia ones instead.
Hope that helps dude. Anything else feel free to ask.