If you have a 32 bit operating system, remove the 1 GB sticks. If you have a 64 bit OS, you can use the full 6 GB. But the answer is a little harder.
If your FSB freq is 333 MHz or less, you can run the slower memory at full speed. The problem then becomes getting all of the memory to run together.
Try increasing the RAM voltage to 2.2 volts and relaxing the timing a little (go from 4-4-4-12 to 5-5-5-15 or 5's-15 to 6's-18).
You are more likely to be successful if you have high quality RAM.
Many thanks to you jsc. I am upgrading my 32bit XP to 64bit Win 7 Pro as well. My FSB frequency is 333 and I do have Kingston RAM. So I am going to try exactly what you have suggested.