wow! this is my first post in like 4 months!. anyway.
Cpu,Graphics card, ram
thats the order of importance when it comes to pj64
1)as long as you have at least 256mb ram, thats enough (more=better of course)
2)cpu speed is very important, more so than the grpahics card.
for instance, i have found that on games like Perfect Dark. I get at the most 5-10fps performance hit MAX when going from no AA to 4x aa! and thats with a gf4 440mx.
that leads me to believe that the cpu is the most important part of the system, which is where your problem lies. (hell i cant get pd to run smoothly with my xp1800)
upgrading the cpu is really the only way to make a significant difference