For those use to Mac OS 9's Get Info window for changing the memory allocation given to programs, you may be surprised to learn Mac OS X does not have the same function for its programs. That's because memory is assigned dynamically to each program, and each program is given its own cubbyhole in memory. In order to do this applications need to be given priority on which ones have access to specific portions of memory. In Mac OS X 10.3's Activity Viewer application and Mac OS X 10.0 through 10.2.8's Process Viewer application you can see which programs have certain priorities. To change some of these you need the shareware called Renicer. For applications in the Classic environment you can still change the memory allocation for them, and you can give the Classic environment up to 512 MB of RAM on its own for these programs. These two knowledgebase articles cover Classic environment's memory allocation:
Mac OS X runs best with at least 1 GB + installed RAM in free hard disk space. If you have 640 MB of RAM in About This Mac in the Apple menu, you need at least 1.6 GB of free hard disk space at the top of the Finder window. Otherwise it will use more virtual memory and slow down because hard drives are slower than physical RAM.