The framework is an apk easily installed:
http://dl.xposed.info/latest.apk (From this thread:
http://xdaforums.com/showthread.php?t=1574401)
GravityBox is here:
http://repo.xposed.info/module/com.ceco.gm2.gravitybox - there's a link to the APK, currently 2.6.9
Just download them both, install the first framework APK first.. then open it. Choose the "Framework" option at the top. Press "Install/Update". It will need a reboot after.
Then install the GravityBox APK.
In your app drawer, go into "Xposed Installer", you'll be back at the same screen as before, press on "Modules", and check "GravityBox". I think it may even need a reboot again, I don't remember.
From there, just go into GravityBox in your app drawer, it has a few good tweaks - the main reason I got this one is because I can make the Navigation Bar smaller so I have more screen space, you can show the menu key etc..
As for the DPI, I just went with this simple app in the market:
https://play.google.com/store/apps/details?id=com.texdroider.texdroider_dpi
I'm currently running at 240 and I've had no market issues, but some apps will think you're on a tablet, so it can look odd. I had no problem with any of the settings I tried so far, so I can't report any real issues with that. I didn't try any crazy values though, I say between 200-300 is probably going to work fine.