since you said this is your first build I will assume your not fully current how it all ties togather. So let me toss out a little explaination to hopefully help you better understand when you put your rig togather and hit that BIOS for the first time.
First off let me just say this, the overall system performance increase you'll get running DDR1066 vs DDR667 is in reality is about 1.5% increase. Yea you heard me right. Less than 2% "OVERALL SYSTEM PERFORMANCE" increase. If you benchmark the memory at the different speed you will see the big difference, but since the memories "effective" speed is way way faster then the FSB of the CPU, the memory is inevitably always waiting for the CPU to catch up. Thus negating its potential back down to the level of memory running at slower speeds.
The times that that paragraph is not true, is in the case of running at a ratio of 1:1 (or 2x memory miltiplier) DDR667 and running a CPU's FSB of 1333 (fsb 333 set in BIOS). Or 1:1 DDR800 and your CPU is running a FSB of 1600 (FSB400 set in BIOS). Or in your case to get the full potential out of that memory you'll need to run 1:1 DDR1067 and your CPU's fsb is 2134MHz (FSB set to 533 in BIOS). Not all mobo's will do fsb 500Mhz. In fact I say the majority wont. alot do but probably not "most"
The simplest explaination I can offer is this. Because DDR is doubl-data-rate, it transfers data on both the up AND down swing of the clock. thuse making its "effective" speed doubled its rated speed. And since the rated memory speed, at 1:1 ratio, is 2X the FSB set in bios, Its effective speed is now 4X the FSB set in BIOS, and that is exactly how the CPU FSB is clocked. 4x whatever you set the FSB to in BIOS. So if you set you FSB to 333MHz x 4 = 1333FSB for the CPU. Just dont confuse this with how fast the CPU is actually running. The fsb of 1333 is just how fast the the motherboard is running (to put it very simplistically).
Dont feel bad though, I'd say 90% of the people out there figured this out AFTER spending big on high end memory over DDR667 or 800. Most people probably never really figure it out. Or even bother to really think about it. If if works they set it and forget it.
The only time that high end memory really pays to have, is if your going for very high overclocks. But until Intel integrates a memory controller on the die as AMD does, the CPU and the video card are what will give you the biggest increase in performance.
So to answer your question: Go with the E6850 and its 9x multiplier, It seems to OC higher, and since it has a higher multiplier than the E6750, it is a bit more flexible in that it can be set to higher GHz without the need of a super high end motherboard. Although the E6750 is a full $100 cheaper, I have been seeing alot of hit and miss for overclocking. The E6750 with its 8x multiplier... if you mobo cant handle over 400fsb then your OC will be motherboard walled at 3.2Ghz. Whereas the E6850, with its 9x multi, on same mobo wil be able to reach 3.6GHz. Now if your buy a motherboard that will do 500+MHZ FSB then you will wont be hindered by the mobo and should be able to find the CPU's max.
I have a DS3 which is supposed to readily hit close to if not over 500MHz. I personally dont know because it doesnt like Allendales which is what I have in my DS3. The P5B Deluxe is another world class overclocking mobo and is also none too expensive, unlike the P5b'S wi-fi brother or the workstation class motherboards.
Wow that got long and I hope you are now absolutely limp with emthusiasm about your new build. hehe