I am planning on purchasing 2 sets of G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL8D-8GBXM, for a total of 16GB. This will be on an Asus Z77 board with either the new i5 ivy bridge or i5 sandy bridge. I wanted to know if running 4 modules at 16GB will maintain 1600 speed or if it will drop. And if it does drop can I raise it back up in the bios? Thanks.
1) the ram will run just fine at it's XMP settings unless you happen to get a bad set of ram (it happens).
2) 16GB is a lot of ram. Seriously, 8GB is overkill for most heavy gaming builds and even most heavy multi taskers. If you are doing production work (HD video editing or other heavy ram programs) then 16GB would be suggested, otherwise stick with a 2x4GB setup.
Running unmatched sets are not guaranteed to work. However the DDR3 processes are very mature and Intel has an extremely good memory controller. I have run unmatched sets on 3 sandybridge platforms. Right now I'm running 8x4GiB at 2133 using just XMP profiles. If you can get 1 set of 4x4 memory that would be better than 2 sets of 2x4. I won't say that it's guaranteed to work at 1600, but it is more than likely that it will work without issues.
Please note that when you do install it the speed may be set to 1333 by default. Even though Ivybridge supports 1600Mhz memory without using an overclocked profile, the memory modules themselves will usually have 1600Mhz+ speeds configured as XMP profiles and not standard specifications. This means that you will have to set the speed to 1600 in the system setup
The XMP is designed for a single kit since you purchased 8GB kits. So although you can try XMP, you may need manual settings to be perfectly stable.
It is not recommended to use two separate kits, but many times people are successful. If they are not able to operate at the rated specifications, tweaks may be necessary, such as raising timings or voltage, to stabilize both kits.