Game requirement: CPU, Card, RAM, Hard Disk!

[Sys14]

My PC's setup:
AMD Athlon XP 1800+
Geforce FX 5900 Ultra
512M Ram
80G Hard disk.

I set my Planetside to: 1280x1024, highest configuration.
The framerate i got is 40-100fps, but, sometimes, the game runs very slow in 3-10 secs. Like when the door opens. And, the continents load very slow now, takes me almost 2-3 mins. And crashs to desktop sometimes. And when i am playing the game, the hard disk LED keeps flashing.

Here is my question:
What should i do to improve it? Is it a hard disk problem? or 512M ram is not enough to meet the requirement for 1280x1024 high?

Someone give me a clue.

Thanks