A few things you'll need to know:
1.) The 533 bus is QDR x133MHz. It uses a 133MHz clock speed, which is usually what BIOS reports
2.) DDR400 runs at 200MHz clock speed. That's a higher clock speed than your CPU bus, but a lower data rate. The best configuration is to match bandwidth between the two, but I doubt your board support the ultra-expensive DDR533 RAM.
3.) The BEST ratio for YOU right NOW is CPU:RAM of 2:3, which is 133MHz CPU bus clock and 200MHz RAM clock.
4.) BIOS on new boards usually refers to the DATA RATE of the memory, but USES the bus speed (which you don't see).
5.) To get into advanced BIOS features on a Gigabyte board, you have to enter a key combination from WITHIN BIOS. I think it's ALT-F1, but I can't remember for certain. You may need those added settings to set your asynchronous CPU:RAM clock.
6.) You won't get a significant improvement from running an 800 bus P4 with a DDR400 single channel compared to a 533 bus P4 with DDR400 single channel. The reason is that the memory is the bottleneck. But the faster CPU with HT provides some improvement in performance even though the RAM performance stays the same.
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