By using renice on the processes related to After Effects and some others related to the system your system will be more responsive. Just launch this script after launching After Effects and render a few frames (RAM preview, render). The script will renice the priority of the After Effects processes, and some others that are related.
And then you may run iTunes, Dropbox, and other programs without problems while working with After Effects.
tell application "Finder" to set theApps to name of every process
activate
set newPriority to "15"
set theApp to "After Effects"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "CS5.5ServiceManager"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "Adobe QT32 Server"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "iTunes"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "Dropbox"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "aeselflink"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "kernek_task"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
on Renice(newPriority, thePID, theApp)
try
set theConfirmation to (do shell script "renice " & newPriority & " -p " & thePID)
on error
set theConfirmation to (do shell script "renice " & newPriority & " -p " & thePID with administrator privileges)
end try
end Renice
[crarko adds: I haven't tested this one. The script lowers the priority of all the processes/applications included in it. The newPriority value of 15 is lower than the default value of 0. Given that renice is just a suggestion to the kernel about how to schedule the process the results can vary, depending on other factors in system. One obvious side effect here is that lowering the priority of After Effects can greatly increase the rendering time of a job. This may depend on the number of processor cores you have available. I guess my point is that doing this may or may not help with performance. It depends on what else your system is doing, and you might modify the script to accomodate that if you feel comfortable doing that.]

