Ok,

i think VladVP is right, there is a big Pile.

1. The whole 128 Bit Coordinates are there to get the Position of an Object in Space.

With 128 Bit there is no need of Overflow Check.

Operations on 128 Bit are Extreme fast.

The slow part on Coordinate Operation will be the Memory Transfer. And the Memory transfer works with 64 Byte! (DDR3 RAM).

2. For objects in the Ship there is only a need of a relative Position.

a 32 Bit Int will be enough vor relative Positions (~ 4,2 Km)

3. The Polygonpositions transfered to the Graphic are relative Positions from the coordinate of the viewer.

those coordinates are with a 32Bit Float precise enough.

4. µm Precision for Simulating to minimize glitches

simple float precision for Graphics to draw

5.

So we could feed the GPU with the last 64 bits of the coordinate integers, and use the first 64 ones as a form of secondary coordinate system, like minecraft chunks. That would be the same as using two integers per axis... which minecraft already does in the form of doubles... so no problems here?

this is a bad idea, because we have to deal with a very complexe Coordinate system. 128 bit <vs> [64 Chunk][64 Offset]

a 128 bit can be used as a simple Integer where all Coordinate relevant Operations (Add and Sub) are VERY fast.

Even multiplication can be done in a few Machnie Cycles.

(Just Dividing would be horrible)

even GPU's can (with the right Code) work on 128 Bit Very fast.

6.

Use GPU as a CoProzessor is a quite bad Idea,

The Simulation engine is very complex and the shaders of a GPU are usualy quite small.

some parts can be implemented very good for GPU's, but then you have to think of the Overhead for Copy the results back and forth.

And especially Servers don't usually have good GPU's

7.

Excuse me, but have you ever heard of MMX and SSE?

SSE has 128 Bit but that's just the register size. Internal it works with 2x 64 Bit or 4x 32Bit.

MMX is deprecated. It was for Multimedia Operations (like saturated MAC or saturated ADD).

and works with 64 Bit. Today every CPU works with 64 Bit. (even smartphones will in 2 or 3 years have 64 bit CPU's)

8.spherical coordinates

spherical coordinates use COS,SIN andy maybe TAN.

for that you have to use the FPU.

No SSE

No MMX

but lame old FPU

and besides that, it would be the worst Nightmare to code.

edit:

9.

Use 80 or 96 bit Coordinates

As mentioned before,

we would safe in the best case at total ~720 Kbyte for 40.000 Objects(which is nothing)

in worst case (and i think the case that is happening) we would save nothing

but in every case we would add a lot of Code Complexity and maybe decrease Performance with 2 non uniform integers.