As far as I have noticed it doesn´t make a big difference.
I tested different graphical and/or ini-settings for hours until I realized that it doesn´t matter if I play on medium settings with shader 3.0 or ultra-low with shader 1.4.
The switch from 1 gig of pc2100 to 2 gigs of pc 3200 brought the much awaited relieve.
I have an athlon 3800+ and a radeon 1600xt with 256mb, the framerates really didn´t change when I lowered the details. Before you waste time like I did just buy the extra ram, it will help for sure. (When you have only 1 gig at the moment)