Some motherboards (like Gigabyte) cannot use some RAM without being overclocked, while others (like Asus) will work with most reasonably priced RAM without further effort.
What I said about video cards can apply to RAM too. Fortunately, newer chipsets don’t seem as affected by cheaper RAM. If you go too cheap (getting a high Cas, lower frequency RAM), your system can suffer. But, you may never notice the difference.
Memory Scaling on Core i7 - Is DDR3-1066 Really the Best Choice?
http://www.anandtech.com/show/2792/13
System Builder Marathon, December 2010: $2000 PC - CPU And Memory
http://www.tomshardware.com/reviews/build-a-pc-overclock-ssd,2792-4.html
Not only are these far lower-grade than the parts we expected from Mushkin, but they’re organized completely differently, with twice the density. That means a module from the current part number 998586 set can’t even be mixed in multichannel mode with a module from the previous 998586 set, even though both carry the same part number. We realize that both versions of this model number are only guaranteed to be stable at the same 1.50 V, DDR3-1333 and lowly 9-9-9-24 timings, but shouldn’t cross-compatibility for modules of the same part number also be guaranteed?
Value Conclusion (Same article)
http://www.tomshardware.com/reviews/build-a-pc-overclock-ssd,2792-19.html
Finally there’s the problem of memory. This is the first time in recent memory that we’ve ran into a hard memory bottleneck in games, and that snag looks like fodder for a completely separate article. Our only excuse is that the memory we thought we would get would have been good enough, but the simple truth is that being cheap has finally bitten this editor in the buttocks. Newegg’s current offering of 6 GB DDR3-1600 CAS 8 for only $95 makes this system look even worse today than it did on the day it was ordered.
I linked the RAM they linked. It’s $70 at this time.
Edit: I should clarify that not all Gigabyte and Asus motherboards are the same across their entire X58 product line. But, I haven’t seen an X58 mobo made by Gigabyte that natively supports PC3 1600 (12800) RAM. And, there are some Asus X58 mobos that need to be overclocked for PC3 1600 RAM to work.