Well it depends on what slot you have next to your x16 one. On some boards there's a PCI, on others a PCI-e.
Er, just read what you said. It will take up only the next slot adjacent to it, so it only takes up 1. Whats nice is that it exhausts the air out of the case and the fan is covered, so unlike a 6800 Ultra that would, IMO take up the next two slots, it doesn't need extra breathing room.
Welcome to the forums, and hopefully you'll be a great addition.