|When RAM of differing specs are installed together, ALL the RAM will run at the settings of the weakest link. So if you have "fast" RAM with timings like 4-4-4-12 & team it up with "slow" RAM of 6-6-6-18, ALL the RAM will run as 6-6-6-18 unless the timings are configured manually. However, CL6 RAM may become unstable if it's run at CL4.|
Same goes for RAM frequency. If you team up DDR2-800 with DDR2-667, ALL RAM will run as DDR2-667. And if you attempt to run DDR2-667 at DDR2-800 frequency (400Mhz), it will likley become unstable or prevent the system from booting.
Your problem isn't timings, it's that your DDR2-800 is only running at 625MHz. Are you 100% sure that all your RAM is DDR2-800? Because DDR2-667 will run at 625MHz when it's paired up with an AMD CPU with 14x multipler. That's the way the memory divider works with that particular config. And since you've said there's no way to increase the RAM frequency setting in the BIOS, the only way to increase the RAM speed would be to increase the CPU frequency. The problem with raising the CPU frequency is that doing so will increase the HT frequency. The HT frequency doesn't require overclocking & should be kept as close to it's default speed of 1000MHz as possible. If the HT bus is too far out of spec, the system may become unstable.
This table lists the resulting memory settings based on the CPU frequency & multiplier settings:
The bottomline is you have a crappy motherboard with a crappy BIOS that has very limited overclocking options. There is no work-around. Either live with it or buy a new board.