Using RAM with different speeds is generally a bad idea. However, if you are able to overclock the RAM to PC3200 speeds, the idea is completely feasible.
The CPU/mobo combo you picked sounds fine. However, if you're looking for longevity, I may recommend a socket 939 CPU/mobo combo. Socket 754 has been discontinued, and currently there are no plans to add more processors. With the new 90nm processors out for socket 939, the option is looking better and better. 90nm processors have increased overclockability and generally run cooler than their 130nm counterparts. This route would be slightly more expensive, however.
How much emphasis are you putting on gaming for this system? This video card you have isn't the greatest for the latest games. It probably won't keep you playing the latest games at acceptable for more than a year, anyway.