Helvetica does not render with antialiasing.
This I think is because Helvetica is not a truetype font and, while many people like the antialiased look in applications, they prefer/are used to non-antialiased fonts on web pages. This is why Helvetica is often the default font when you first install Mozilla or Konqueror. In this way you are able to render web pages non-antialiased with antialiasing enabled.
Further investigation will show that Helvetica is not located in the true type folder but in the 100 or 75 dpi font folder associated with the other X window fonts. Fiddling with your dpi values and listing 75 or 100 dpi first in xorg.conf may present you with different results.
I have a rather rambling thread HERE
that attempts to explain how to render nice non-antialiased fonts. This will have no effect on making Helvetica render antialiased though because it is not a truetype font. Not what you are asking, but you may get some background info from there if you are interested.