In answer to your question, just about everything.

Your hdd's are ok, i had a p3 866 with 512mg ram and i would average about 8 - 12 fps on the ground at a busy airport with most of my settings turned way up.

I would go for new mobo and chip and even a gf 3ti or a gf4 ti card.

i upgraded my p3 to an athlon xp2000+ and new mobo with ddr ram for $400(AUS).

The gf4ti 4200 is a good card and not too badly priced at the moment.

If you cant afford these upgrades then start with another stick of ram - even go as high as getting a stick of 512mg 133 sdram.

Your not going to get the fps that i think you are hoping for without a major upgrade of mobo, cpu and graphics card though.

Just play with the settings - turn up what you want heaps of and turn down what you don't want.

Hope this helps

Jordanick