Karnak, at this point there really aren't any *significant* upgrades to perform. Sure you could buy the XP 2600+, get a new KT333 based motherboard, and upgrade to pc2700 RAM. You could then overclock both your cpu and your graphics card. All in all, you *might* notice a difference of a few frames per second. Sure, it looks nice that the new Radeon 9700 can play quake3 at over 300 frames per second...but one thing to remember...your eye can't go over 30 fps.
The real benefit to the upgrades is how the game handles when it has a bucketload of stuff to do...like process smoke effects from multiple sources, vertex shading, anti aliasing, z-buffer clears, etc. That is where the powerhouse graphic cards shine...and your 4600 is no slouch.