Most motherboards run the RAM speed synchronously with the CPU’s FSB. If the FSB is faster than that, it will run it slower. It sounds like you have a 333MHz FSB with your CPU, and the motherboard is matching it to that. The other possibility that I can think of is that the chipset doesn’t support (officially) past 333MHz, so it won’t run the RAM at that speed.
You may be able to run the RAM at the full 400MHz by going into BIOS and adjusting the CPU FSB:RAM ratio from 1:1 to 4:5 if that’s an option. The only time you don’t want to do this is if your motherboard runs an nForce 1 or 2 chipset, because the performance goes down when running asynchronous FSB:RAM ratios, even if the memory is faster. No other chipsets that I’m aware of have this caveat.
Okay, you may just have to manually set the RAM speed in BIOS then. It sounds like the BIOS went with a ‘safe’ option and ran the RAM at a slower speed. The 430 is an nForce 4 series budget chipset, alongside a few others, but you’ll be able to run in at 400MHz. You’ll need to go into BIOS and make the adjustment and then save your settings. Without knowing the motherboard, I’m not sure exactly where to look, but you can check the Frequency and Voltage Controls section in most BIOSes for that info. Also, your motherboard manual should give that information.
Now, if you have another stick of RAM that’s PC2700 (DDR333), then you won’t be able to run it at PC3200, unless you have some really high-end stuff.