I assume it's P35-DS3L? There's a P31-DS3L as well which doesn't take 4 sticks of dual sided ram.
Which CPU do you have and at what multi? That determines whether you need high fsb to reach 3Ghz and thus determine whether you need high speed ram to keep up with the FSB.
A PC5300 DDR2 667 is good for FSB333 (1333Mhz effective), which will take you to 3Ghz if you have 9 multi CPU (E2160,E4300,E6600,E6850,Q6600).
However it is likely that it will overclock well beyond PC5300 anyway. But with the prices of RAM so low it's worth getting a PC6400 RAM, PC8500 stuff is unnecessary unless you're looking to overclock one of the new E8*** series C2D to the max.