ATI has released both a PCI-E and an AGP version of the HD4850. The easiest way to know is to take out the graphics card from your computer and look at the images Franco B posted, but it's most likely that you have a PCI-E card because AGP cards really aren't in demand anymore because it's old technology.
It seems like you've already downloaded the PCI-E drivers and it's working fine, so I would assume that's the type of card that you have.