Erm with regards to 3D rendering the LGA1156 with a Core i7 860 vs a Core i7 920 on an LGA1366 show the 860 as being just as quick if not quite a bit faster then the 920 according the
review that Maziar posted. The same goes for
video encoding with the 860 performing just as quickly as the 920 if not faster just by a whisker. I'm comparing the 860 and 920 since they are similarly priced where I live (UK).
Gaming performance is harder to judge since it's the video card that does most of the work these days but you still need a good CPU to avoid bottlenecks, the 860 has a default clockrate of 2.8Ghz which is slightly higher then the 920's 2.66ghz however that's not where the advantage is for the LGA1156 chip. The new Core i7's have a very aggressive Turbo mode which automatically overclocks the CPU when it's pushed, the 920 has a Turbo mode as well but it's no where near as fast as the new chips.
The argument of being limited to x8/x8 when running crossfire on the new Lynnfield chipsets is a none argument as well, just reads Tom's review on the Radeon HD5850 running on x8/x8 which shows there is no bandwidth limitations. Taking this a step further over at
TechPowerup they found even if you go down to x4 the performance drop isn't that great.
The best arguement you can make for getting a LGA1366 setup over a LGA1156 is that you will have a board they 'may' run Intel's up and coming Gulftown 6 core CPU's. If this is plus for you I should let you know that any 6 core monster is going to cost $1000 minimum and I wouldn't expect to become affordable for at least 18 months or two years afterwards.