My system isn't all that leading edge. Nice mobo (A7N8X deluxe), then a very middling XP2600 processor which runs at 1.9GHz. The video card is also middling - ATi Radeon 9600 Amateur, and I have the latest drivers which were published this month. And I'm getting 60-75fps normally. That's with antialiasing maxed out at 6X and anisotropic filtering at 4X. Those settings make the graphics much sharper, and stop the trailing edge of your wings from looking like hacksaw blades.
Also, if your fps is jumping from one extreme to the other, you probably don't have Vsync turned on. It should be ON.
How do I know all this? I'm no brainiac, but I DO read the forums to see what other guys are doing, and discuss these matters. There's a wealth of knowledge in there. Try it sometime.
Now for the cost of upgrading - if some of that gun money were instead spent on useful things like computer upgrades...

Slapshot & Icer.