You won't have to buy a new MB.
http://www.gigabyte.com.tw/News/Moth...px?NewsID=1423

It will work, but how well it will work noone knows at this point.
I also hope ACC solved some problem with the 65nm parts that isn't a problem anymore with their 45nm parts, but I think a 10% difference between highest overclock on SB600 and SB750 is a more realistic expectation at this point.