Current 975 boards need a couple extra capacitors (2) and a few resistors (10) to make 'em work. Intel also sells chipsets so they're gonna want you to buy a brand new 985 mobo, right?
Here's the doc describing the differences:
ftp://download.intel.com/design/chipset ... 015801.pdf
So you can either buy 975 rev. 2 or pay the extra $20 and get the 985. Intel is notorious for socket changes--it's one of the reasons their income is higher than AMD's. It's also the reason they're not my personal favourite.
Quite frankly, 3Q and 4Q 2006 is Intel's year IMO. 2007 is where things get interesting again.
So that means it's time to buy bargain 939 equipment.

I've already got my 3ghz Opteron--now I just need to decide on a RX480, RD480, or RD580 mobo.