Power Mac - Power Mac G5 Memory Question / Issue

I've got a G5 1.6 running OSX 10.4. She's maxed out with 4 gigs of RAM. I've also recently installed an nVidia GeForce 6800 Ultra DDL video card. Last night, I was watching Gran Torino on VLC (which commits the file to memory, albeit "inactive" as it plays), running Seashore, Celestia and Mac The Ripper in the background. As the movie approached the end, I switched over to a separate desktop and attempted to open Limewire. The program would partially open and then quit, however in the Activity Monitor it wasn't in a kernel panic, rather it stopped using processor time and simply did nothing. I forced it to quit, then opened it again, and the same happened. Then I noticed that my RAM was maxed out. I realize that there are options such as optimizers and closing open programs, but doesn't OSX have a provision to wipe out "inactive" memory and allow new programs to be opened? Are my sticks bad, or is this a memory controller issue... or... help... please?

Gracias in advance.