I would narrow it down to either RAM or the processor.
With a new Athlon, you would get about 50% more clock speed, and with a new stick of RAM, 100% more memory.
If I were you, I would consider whether or not being able to run more processes would help more than being able to run one process, such as a game, a lot more efficiently.
Intel P4 2.8GHz H/T
Windows XP Professional