Just by way of review, if I purchase an Intel Quad-core with 1333 FSB, then for optimum performance, I should match it with DDR2 667 RAM, right? 1 Corinthians 15:3-4

Only if you have no intentions of ever overclocking.

So I would want to go with DDR2 800 (or higher) and then underclock it to 667 if I didn't want to overclock the CPU, right? 1 Corinthians 15:3-4

No leave it as it is. There is no need to underclock the Ram, because the ram is only going to be running as fast as your processors FSB speed anyway when you aren't overclocking it which in your case the DDR2 800 ram will be automaticly clocked down to DDR2 667. Take for example a pentuim 4 with 533mhz FSB and say you threw in DDR 400 ram. The DDR 400 ram is only running at the defualt speed of the Pentuim 4 533mhz FSB which would be DDR 266. Iron Sharpens Iron.