Using XBench and CineBench, it's apparent that MacBook's are in fact faster when using two DIMMs of the same size than using a single bigger DIMM. The configuration difference seems to have the biggest impact in graphics performance. Here's a link to the XBench comparison.
For reference, here's my PowerBook CineBench 9.5 results:
Processor : 1,5Ghz PB G4 1,25GB RAM
MHz : 1500
Number of CPUs : 1
Operating System : 10.4.7
Graphics Card : GeForce FX Go5200
Resolution : 1024x768
Color Depth : 24bit
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Rendering (Single CPU): 152 CB-CPU
Rendering (Multiple CPU): --- CB-CPU
Shading (CINEMA 4D) : 161 CB-GFX
Shading (OpenGL Software Lighting) : 485 CB-GFX
Shading (OpenGL Hardware Lighting) : 1015 CB-GFX
OpenGL Speedup: 6.32
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Next we have the 1x1GB config 2.0 Ghz MacBook:
Processor : MB 2GHz 1x1GB
MHz : 2000
Number of CPUs : 2
Operating System : 10.4.6
Graphics Card : Intel GMA950
Resolution : 1280x800
Color Depth : 24bit
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Rendering (Single CPU): 307 CB-CPU
Rendering (Multiple CPU): 582 CB-CPU
Multiprocessor Speedup: 1.89
Shading (CINEMA 4D) : 336 CB-GFX
Shading (OpenGL Software Lighting) : 1068 CB-GFX
Shading (OpenGL Hardware Lighting) : 969 CB-GFX
OpenGL Speedup: 3.18
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I'll post the 2x512 Results soon…