I dont understand, I thought that "LGA 1155" was the type of socket? They are both LGA 1155 but the CPU wont work?
Yes, they're both LGA1155 but some motherboards may not be compatible with some series of CPUs. I believe it's because of the software on the motherboard that doesn't recongnize the CPU, although I may be wrong. Either way, I wouldn't buy it because it's not one of the supported CPUs.
To do crossfire or nvidia sli you will need a larger PSU and a different motherboard. It wouldn't really make sense to do with that CPU anyway. Since you're on a budget I'd just go with what you have and not worry about XFire/SLI.
For Diablo just to be sure, you might pay a bit more and get a 6850. I'm not sure where exactly your CPU would start holding back the GPU but its probably somewhere near there. But your original match of a 6770 makes a decent budget gamer out of it. Since that original one ran out of stock you can just find another 6770 from another brand and it'll perform the same.