How big of a difference does cast latency make on ram? (i.e. can I get more speed with 2133mhz with 9 cast latency, or 2400mhz with 11 cast latency?
CAS latency is very important. The lower the CAS latency, the less time it takes for your RAM to start actually doing anything. It's essentially a delay. I have a set of G.SKILL Trident X myself that shipped at 2400mHz CL10, which is as high as I'd be willing to go. I don't do any serious editing, and mostly (and increasingly less) just casual gaming, so I don't need the super fast frequency, and the exceedingly high CL doesn't hurt me.
If you are getting a 2133mHz kit, for gaming or what have you, get one with a CL of 9. That keeps you a little above average in the ratio of frequency to CL, which is a system that I made up, and is probably not at all the right way to measure t he relationship, but hell, it is easy. 2133mhz / CL9 = 237. The standard is 1600mHz CL9, which equals 177.77. You wnat at least a ratio of 177.77, and preferably higher, for most situations. If you are doing serious editing, get that CL down as far as you can possibly do it. There are lots af 1600mHz CL6 kits around on eBay, but they aren't made anymore, so prices vary. If you are gaming, CL doesn't matter nearly as much, so more speed is generally better.