OK, basically whats happening at the moment is that your processor is processing the graphics faster than your video card can handle (way way faster). Therefore you have a huge bottleneck. To solve this you turn the detail in the game down to avoid the bottleneck. The GF4 you are planning on buying is designed for your processor speed, so no bottleneck. The result is you'll be able to play GP4 at full detail and a very high FPS. Its well worth the upgrade!
As for the Win 98/XP thing, I personally dont see the point in dual booting. XP runs fine with all my games and the OS is a hell of a lot better than 98, which will just take up HD space which you could use for MP3's
Or GP4 addons