The game I was most curious about was MW2. I has trying to setup a split screen on one xbox with a third player on another xbox on the LAN. Didn't seem to work.
I did notice playing 4 player split screen on MW3 on one xbox seemed to slow the game a bit.
We ended up playing black ops on the PS3 doing a lan party, 2 on one PS3 and one an another. Unfortunately, I was playing with my youngest son, it didn't take long to figure out he looks at my screen:-)