Thank you very much for the replies. I was already aware that the newer, faster RAM would clock down to 667MHz to support the motherboard.
I was looking at local prices and 4gb (2gb x 2) of PC8500 was cheaper than 4gb (2gb x 2) of PC5300/5400.
I am hoping for the most bang for the buck on my aging 2yr old PC running Windows 7 32bit. Even my work issued laptop has a faster Core 2 Duo processor and 4GB or RAM!