Memory in early 2008 Mac Pro
Considering you have Pairs of equal memory (2x1G, 2x2G and 4x4G) the best option is to install them all (that's a lot of RAM!).

Early 2008's Mac Pros use the full memory bandwidth when modules are installed in Pairs (like you have). But their location in the risers (where you install the modules) is important. Therefore, you should follow what the manual says.

Since you're going to fill all the 8 slots, you can go ahead an install them in pairs one next to the other. You have 2 risers, so here's how you should put them:

TOP RISER

SLOT 1: 1x4GB

SLOT 2: 1x4GB

SLOT 3: 1x4GB

SLOT 4: 1x4GB

BOTTOM RISER

SLOT 1: 1x2GB

SLOT 2: 1x2GB

SLOT 3: 1x1GB

SLOT 4: 1x1GB

The graphic explanation for this can be read in the Early 2008 Mac Pro Memory Manual, located at manuals.info.apple.com.