I am looking to add memory. I already have 2x2gb sticks installed. When I buy more memory is it import to match the voltage, cas, and timing of memory across a few memory modules in a computer? Or is the speed and type the major concern?
Yes the new RAM should have the same voltage,timings and speeds.You can sometimes get a different RAM and tweak its settings to have the similar specs of the older one but it may become unstable,therefore its recommended to get a RAM with same specs.
The Voltage could be a problem ! -- you can only set a single voltage for all of the RAM so with a .2V difference in the rated spec for the 2 modules you might run into problems (I say might because some modules will work with higher voltages while others could have problems and some can run on lower voltage than their spec while others will be unstable with less than spec !) -- SO it really depends on the modules you get whether it will be a problem or not - trying to run the old set at a higher voltage might work but could overheat them while trying to run the new set at a lower voltage may cause them to be unstable and cause the system to blue screen.
Really would be better to get some with matching voltage if you can to avoid any possible trouble -- that said many times the modules will work together at either voltage setting but unless you are willing to play around with different setttings to get them stable you would be better off getting some that are more closely matched ( the Kingston modules probably neeed the extra .2v of power to remain stable at those timings so to run at the lower voltage may need to have the timings loosened.
In Summary you might be able to get the new modules and put them in and it will all run fine but there is also a chance that it will be a problem so best bet is to find some rated to run at the same voltage !