|It may be a chipset limitation. IIRC, the stubborn 945 could only handle up to DDR2-667. |
I wouldn't worry too much about it; you're not losing much performance by running it 3:5. In fact, if you use the integrated graphics, you should run the memory as fast as the chipset lets you.
Socket 939 | Dual-core Opteron 185 @ 3.2GHz | 4GB | 1.0TB | Win7 x64
GeForce GTX 460 1GB | X-Fi Ti Pro | A8N32-SLI Deluxe | Antec P182