Best way to increase frame rate to suit less powered platforms?

My platform is:

CPU: XP 1800+
CHIPSET: KT266A
RAM: 512 MB

My video card is:
GF 4 Ti 4600 128 MB Ram

Obviously, the video card outperforms the platform easily. My question
is; when selecting video options in games, what are the things that
depend on CPU, as opposed to video card that should be modified so as
to produce the best frame rate?

For instance, I know the resolution itself depends more on the card
than the platform and that reducing it may not produce any
improvements. I would expect the same for texture detail levels and
anti-aliasing (although anti-aliasing will be card dependent and will
generally have an effect). But do things like shadows and smoke
matter? What other sorts of things should I look for so as to best
match my video card and platform to produce best quality while
retaining highest frame rates?

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