Because I could get in deep sh*t if I sold it. I'll leave it at that
From what I'm discovering DDR3 is at most *marginally* faster than DDR2. Most of the time the difference is negligible. Unless you get crazy expensive very high speed DDR3 RAM then you might see a few percentage points.
Two articles, one from anand and the other from xbit labs both conclude that at the same Mhz there is very little difference. [Note: both articles are from 2007 when DDR3 launched. Speeds topped out at about 1333mhz].
http://www.anandtech.com/memory/showdoc.aspx?i=2989
http://www.xbitlabs.com/articles/memory/display/ddr3.html
Then I saw this article from Anand in June of this year "Memory Scaling on Core i7 - Is DDR3-1066 Really the Best Choice?". Up to 40% better in synthetic benchmarks, but it didn't translate into realworld applications.
http://www.anandtech.com/memory/showdoc.aspx?i=3589
To quote from the article
"Once again, as we moved to real-world applications, those impressive synthetic benchmark improvements did not translate into results that would justify spending three times as much for a memory kit for most people. We had mixed with certain applications like WinRAR producing a 20% improvement from DDR3-1066 C7 to DDR3-1866 C7 while several applications showed minor performance improvements under 2%. If your primary job is to compress and archive files for a living, then the expenditure for fast low latency memory is justifiable."
If I was building an i7 platform, then yes DDR3 all the way. Or an AMD AM3 socket build. Otherwise, nice fast DDR2 seems the way to go. I'm still going to check out your recommendations and see if that might be a better way to go. But a few people I've spoken to who own a qx9650 (which my qx9750 is very similar to) swear by the Gigabyte board.
