Installation of a Radeon 4870 Graphics card
If it's the 3,1 [Early 08] the connectors are just under the front fan assembly, near the top. Cables should already be in situ &amp; just need disentangling. 

Not sure on the earlier machines, but they haven't moved far from there on the 4,1 so it might be reasonably consistent across models

Picture, courtesy of iFixit of the 3,1 location...