Using a Gigabyte MB and cooling with water my i7-950 is rock stable at 4.3 with 24GB of 1600 G Skill RAM running at 1624; I have CPU turbo disabled and RAM XMP enabled and running at 1.5 volts.
Based on my clocks (BCLK 203, CPU 21, turbo off, QPI 36, MEM 8, and Uncore 16), I would suggest trying the following to reach 4.0 with your 1600 rated RAM:
If that is not stable for your Asus, Try lowering BCLK until go get something that is. These settings would put your RAM at 1520 which would certainly be close to their maximum potential.
OC'ing can be time consuming; if this "formula" does not work for your system, then you really do need to go with the tried and true methodical approach.
Regarding which is more important for PPro I would say CPU speed is much more significant than RAM speed based on runs that making with Harm and Bill's ever so useful PPBM5 benchmark along my journey.
On another note, Cristobal on this forum informed me about "INTELBURN TEST" (Google it), which was a more rigerous torture test for stability than the ones that most people use (memtest, 3DMark, wPrime, etc.).
Good luck and do report back what works best for you in the end; it would be nice to report how your drives are set up too along with your PPBM5 results as well.
The reason why you get your ram @ 980 is because you have to change the multiplier of the ram.
If you block is at 160 and you cpu multi is @ 25 you get 4ghz
The same whit the ram: your block is @ 160 and ram multi is @ 6: 160x6= 960mhz
if you ram multi=8: ram speed=1280mhz
if you ram multi=10: ram speed=1600mhz etc..
You have to make shure the latency of the ram and the voltage are ok.
PD. Read the O.C. Guide harm made that will helyou to understand a bit more about O.C.
"IF I am stucked in making choices and can't have it all (CPU 4.0 and 24D RAM-1600), WICH IS THE MOST IMPORTANT: CPU SPEED OR RAM SPEED?"
CPU SPEED. But as I just wrote you, you only have to change the RAM multiplier to 10 to get the 1600hz.