Can't Open Applications on Old Mac (OS X 10.4)
You can look up the specs on a particular Mac at Everymac.com, which would tell you whether you can add RAM, and what the maximum is. The Everymac site allows you to look up a Mac by serial number, which you can get from the Apple Menu, About This Mac, then  "More Info...", which opens System Profiler. It will also tell you the Machine Model, which might be something like "PowerBook6,8", which can also be used to look up a given Mac's specs.

If you go down to "Memory" in the System Profiler list it will tell you the type of memory installed.

You also might want to watch Activity Monitor as you open the program to see what it does with memory usage. Can you open the program without loading the project you are working on, to make sure the program is OK, then try to open the project?