How important is CAS vs. memory speed?

if both RAM same Clock Speed and just different CL , and used default system i would go muskin . it will different when you ran OC ... corsair is choice

when you say considerably faster,, do you mean noticeably in real world? you may never really notice unless you ran some tests, that being said, the lower the CAS when memory runs at same speed, the faster the memory.
the higher end memory (1600+) with higher latencies are sometimes slower than the slower memory because of the high CAS numbers
i wouldn't worry too much about the CAS in the end unless there is some big reason, if same price go with lower CAS if you are worried about price, save some money and go cheaper.

Corsair CL8...what speed?
Mushkin CL7..what speed?

And more important, what CPU?

Memory speed makes more of a noticeable performance impact than timings do.

1600 CL9 versus 1600 CL8 -- the CL8 will be faster, but only barely and not noticeable in real-world programs/games.

1600 CL9 versus 1333 CL7 -- the 1600 will be faster, and you would probably be able to actually tell the difference in real-world programs/games.

I just searched for both 1600 Mhz speeds. That's what my current memory is. I can buy faster speed memory if that's more important. It will be with an i7 2600k and I do like to overclock things. I currently have an i7 920 and my memory is "overclocked" to 1600 mhz which is the rated memory speed, but obviously on 1366 platforms everything defaults to 1066 until you turn it up.

I'm just toying with the idea of building a new system because I have the chance to get SSD's and some memory paid for by school for "research uses" because we deal with large data files. I also have a friend looking to buy/build a computer soon so I have the opportunity to sell him my mobo/memory/cpu and upgrade for little money out of my pocket so I can take advantage of Sata III with SSDs (I went 1366 before Sata III existed).

I currently have Corsair XMS3 memory and it's been great. I always said my next memory would be Corsair Dominator memory, but you can only buy Dominator dual channel at CL8. I guess my ultimate question is: is Cas 7 or lower worth deviating from the Dominator memory? It doesn't sound like it is from what I'm hearing.

No it isn't. However, I'd recommend Corsair Vengeance instead of Dominator. All new systems recommend 1.5v for RAM voltage (especially for Intel CPUs).

For your system (i7 920) you would see not real world performance increase going from 1600 CL9 to 1600 CL 7. There are a couple of software apps that may see a slight performance gain. I'm guessing that your Ram is 1.65 V and is OK for the i7-920 system.

However for the newer SB CPUs (Ix-xxxx) as leaps-from-Shawdow indicated the Ram should be 1.5 V. Intel's specs for Ram is 1.500 ± 0.075V. I,m running my DDR3-1600 CL7 @ 1.600, just over the Max.
For SB systems the sweet spot is DDR3-1600 CL9 @ 1.5 V. However in the test they found only a slight diff between DDR3-1333 CL 9 and DDR3-1600 CL9 - Only worth selecting the higher end Ram if Price is within $15 of the lower end Ram.

Added a reference.
Going from DDr3-1333 CL7 -> DDR3-1600 CL7 ONLY results in a 2-> 4% increase in most cases (For graphics and gaming - NO diff)
. It normally takes at least 10% increase to notice any real world change.
Ref: http://www.anandtech.com/show/4503/sandy-bridge-memory-...