Photoshop uses RAM differently as it also uses a swap disk/scratch drive - no idea about Alias but I imagine it is similar and something to do with the paint module. Rule of thumb for any rendering app - get as much RAM as you can fit, and use it. If you are using a 32 bit version on Windows the maximum RAM Keyshot can access is 2GB (rule of thumb). If you are using 64 bit then it can access a lot more. On the Mac side (is the Mac side 64 bit yet?) you can access a bit more, but in general the Mac handles memory more efficiently than Windows so you can load larger models.
Personally, I don't give it a thought, but I don't run any other apps while using Keyshot. Once I get a new shiny 64 bit workstation I will pack it with 16GB of RAM and run every piece of software I have at the same time - just for the hell of it!