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 updates 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.
Ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can very sensitive to this.
It is better to get what you need in one kit.
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.
Fancy heat spreaders are mostly marketing too.
Only if you are seeking record level overclocks should you consider faster ram or better latencies.
Without knowing your motherboard and how many gb you need, I can't be specific.
If you are looking for a 8gb kit of 2 x 4gb for a sandy build, look at these for $69:http://www.newegg.com/Product/Product.aspx?Item=N82E16820231311