I found a dpi font option on the FreeType/2 configuration, is that one?
No. Do not mess with that setting unless you have some extremely specialized needs (I don't know what). It won't rescale text in Presentation Manager, all it does is shrink or inflate glyph vectors within the fixed bounding boxes defined by PM, so you'll probably end up with either tiny spaced-apart letters, or distorted and cropped letters in spaces too small for them.
Honestly, I don't know why this feature was included in FreeType/2 originally, but Michal put it in for (I guess) some reason, so I left it there. If I ever finish and release version 2.x, that setting will be eliminated. I can't imagine any use for it.
Any tips on some good values for HD resolution?
Use the Scheme Palette (not the Font Palette). Set your menu, window, and desktop icon text fonts to larger values, and use Alt+Drag to set it system wide (it may take a couple tries for it to 'take').
Unfortunately a lot of programs (including almost all XWorkplace settings pages) hard code the font to 9.WarpSans and that cannot be changed. (The Styler substitution feature can help with dialogs, but not in all cases.)