How can I get Preview to open up in the Foreground?
Auto-opening a downloaded file from a browser is done on pretty much a browser-to-browser basis. Since you asked about Chrome, here's the relevant blurb from Google's Docs: 


  Automatically open certain types of
  files
  
  If you want certain types of files to
  always open after they're done
  downloading, click the arrow next to
  the file button in the downloads bar
  and select Always open files of this
  type. To prevent potentially malicious
  files from automatically downloading
  to your computer, this option isn't
  available for executable filetypes,
  such as those with .exe, .dll, or .bat
  extensions (for Windows), and .dmg
  extensions (for Mac).


Personally, these features weren't visible when I tried it, but then I hardly use Chrome other than for the odd web site test.