Your FSB is quad-pumped, so to run RAM at a 1:1 ratio, you'd actually have to run it at 266MHz (533MHz effective, or PC4200 DDR2). I'm not sure, but I think you'd be better off running PC6400 DDR2 at 400MHz (800MHz effective) unsynchronized.
I think you should run ddr2 3200 duo channel or ddr2 6400 single changle for core2duo E4xxx series and ddr2 533mhz duo channel or ddr2 1066mhz single channel for c2d E6xxx except for E6x50s. In C2D E6x50s, you need to get DDR2 5400 or 667mhz in duo channel or 1333mhz in single channel. You could also use ddr3 if you want on p35 chipsets, but it think it would be very expensive and underperform fast ddr2 module.
E6x50 is 1333mhz FSB. Its not out until July 22nd which is the price drop. DDR3 is new type of ram supported by P35 chipset. It's true that intel uses quad pumped bus, but you match the ram with the quad pumped bus not the basic FSB. E.G 1:1 Ram ratio for E4300 is 200mhz x 4 ( FSB) : 200mhz x 2( duo channel) x 2 ( DDR2 ) THis is the case for DDR2 3200
The difference between PC3200 and PC6400 isn't a lot if you're not overclocking. In fact PC3200 might even be faster at 1:1 ratio because you could set the timing to 184.108.40.206 or something. 1:1 is the most effective and most efficient and most money saving to achieve great performance because neither RAM or FSB bottleneck each other. If The speed is faster than 1:1, the data from the RAM is too much for the FSB thus the extra speed does not effect performance much. On the other hand, timing can change performance by a lot because the faster the timing, the faster the chipset's able to deliver Data from Ram to CPU. BTW, PC3200 is nearly half the price of PC6400. The best way to get great performance is buy PC5400 (667Mhz) and downclock it to PC3200 @ lowest timing. Crucial Rendition Ram (667mhz) 1GB is 29.99 @ 4,4,4,12 timing. I think its cheaper than buying PC3200@ 3,3,3,8 timing which is around 50$.
The difference between PC3200 and PC6400 isn't a lot if you're not overclocking.
First, PC-3200 and PC-6400 both refer to DDR RAM, not DDR2 RAM. Assuming you mean "PC2-3200" and "PC2-6400," reported differences range from a few percent to about 20%, depending on the app.
In fact PC3200 might even be faster at 1:1 ratio because you could set the timing to 220.127.116.11 or something.
Not true with real-world hardware.
1:1 is the most effective and most efficient and most money saving to achieve great performance
This is a common misconception.
... because neither RAM or FSB bottleneck each other.
True with respect to sustained throughput (i.e. consecutive memory addresses being accessed), but this ignores the influence of latency, which becomes especially important as memory accesses "jump around" in the address space. For example the MadShrimps test data (http://www.madshrimps.be/?action=getarticle&number=1&ar...) shows no big change going from DDR2-533 (1:1 ratio in their setup) to DDR2-667, but a noticeable improvement from continuing on to DDR2-800. Presumably, the slight latency decrease in going to DDR2-667 pretty much offsets losing synchronization between the memory bus and FSB; increasing the memory bus speed even more to DDR2-800 reduces latency further, resulting in performance increases.
BTW, PC3200 is nearly half the price of PC6400.
Current newegg prices are roughly $40 for either, for 2x512MB DDR2.
... The best way to get great performance is buy PC5400 (667Mhz) and downclock it to PC3200 @ lowest timing.
Now this is just silly.
The answer to the OP's question is that you should not run the RAM slower than 1:1 (DDR2-533 in dual-channel mode for 1066MHz data rate FSB CPU). Running one "notch" faster (DDR2-667 in this case) won't boost performance, but running two or more "notches" faster (DDR2-800 or higher in this case), will boost performance a bit.