[Coda]

Physical RAM *is* a big deal, and CPU load *isn't* until you hit 100% on a core.

If you run out of physical RAM, your system has to use your hard drive as additional RAM. Now, if you've got some memory-hungry program in the background not doing anything, the OS knows to stick that program in the slow disk-as-RAM region (known as "swap space") until you start doing something with it.

But if Firefox is hogging RAM and it's trying to do something, if it tries to pull more physical RAM than you have available, it's going to start "thrashing" -- that means that the operating system will be switching stuff from physical RAM to disk and back because data in swap space can't be used by programs.

Once it starts thrashing, it's basically game over. Sometimes if it's not thrashing too bad you can just wait it out, but if your disk is sitting there churning hard for more than a few minutes it could take a process that normally only runs for a few seconds not finish after several hours. (Anecdote: One time I tried building a large program on a computer with 1GB of RAM. On my laptop (4GB) it took about 45 minutes. On the other computer, I had to kill it after two days of not reporting any change in progress.)

As for CPU time, if all of your CPU cores are at less than 100%, it means that your computer is spending time doing nothing at all -- it even lowers the electrical power to the CPU, because you just don't need it. Theoretically you shouldn't really even see a difference in performance between 25% and 75% CPU usage. Once one of your CPU cores hits 100%, then the program running on that core can't go any faster or do any more work, but other programs can run on other cores. Once ALL of your cores hit 100%, then NOTHING can go any faster or do any more work and trying to do more requires taking time away from something else.

Edit: With a five-year-old computer, there's an off chance you may only HAVE one core but I think by 2008 most new computers were shipping with two.