Nope, video card is more important for gaming. Go with the 6800GT. Make sue you get the right version though, AGP or PCIe16, dunno what 754 motherboard you have.
Not ALL highest, on all highest I've seen FEAR (on some machines) jerk whenever u go to a new location (due to inadequate ram), and i'm talking the people had 1GB (so more than 1GB is recommended), also on EVERYTHING high the 7800GT+ have been taxed... so not all highest, but it'll look sweet.