You want documented ram compatibility. If you should ever have a problem, you want supported ram.
Otherwise, you risk a finger pointing battle between the ram and motherboard support sites, claiming "not my problem".
One place to check is your motherboards web site.
Look for the ram QVL list. It lists all of the ram kits that have been tested with that particular motherboard.
Sometimes the QVL list is not updated after the motherboard is released.
For more current info, go to a ram vendor's web site and access their ram selection configurator.
Enter your motherboard, and you will get a list of compatible ram kits.
While today's motherboards are more tolerant of different ram, it makes sense to buy ram that is known to work and is supported.
As to ram speed,
The current Intel nehalem and sandy bridge cpu's have an excellent integrated ram controller. It is able to keep the cpu fed with data from any speed ram.
The difference in real application performance or FPS between the fastest and slowest ram is on the order of 1-3%.
Synthetic benchmark differences will be impressive, but are largely irrelevant in the real world.
If possible it never hurts to get memory that is on the QVL, but in this case the QVL doesn't list a single 8GB kit, so unless you want to downgrade to 4GB then I wouldn't worry about it. Personally I don't let what is or isn't on the list factor in very much. I just built a system with a ASRock H61M-VS board and 4 GB of RAM that wasn't on the QVL and it worked out fine. There are no guarantees though, even if the RAM is on the support list.