RAM underclocking in BIOS?

I recently replaced the Athlon 64 3200+ CPU in my Asus A8V Deluxe with a dual-core 4600+ (I was lucky to find one of these nearly-extinct Socket 939 creatures). I have the latest BIOS (1017), which I allowed to configure its automatic settings (no overclocking or plans to do so). When I got everything set up, I ran CPU-Z. It told me that my RAM was running at 166MHz. It also confirmed that my 2G of Kingston ValueRam PC3200 (4 pieces of KVR400X64C3A/512) was rated for 200MHz (DDR400). So I went into BIOS setup, changed "Memclock Mode" from "Auto" to "Limit," and set the speed to "2:1 (DDR400)".

CPU-Z now shows a memory frequency of 200MHz, and Windows seems happy with it. The other memory parameters that CPU-Z shows haven't changed. I believe that I've merely reset the RAM speed to its correct full rating. But could there be a good reason for the BIOS apparently underclocking the RAM in its automatic configuration?

Some MBs are only rated for a max speed of DDR-333 when 4 DIMMs are installed, but will run at DDR-400 when 2 DIMMs are installed. Your system sounds like its working OK, but run memtest86+ and something like Orthos, dual PRIME95, or other CPU/RAM stressor to make sure.