At present the AppleScript support in Chrome is very limited (beyond the standard GUI scripting native to Mac OS X) but is expected to improve in Milestone 5. (Release date unsure) See http://code.google.com/p/chromium/issues/detail?id=27468 for more details.
So at present there is no way to reliably create a new window if Chrome is already running. If it's not running it's easy to use:
tell application "Chrome"
Of note running:
open /Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome "URL HERE"&
is a great way to get multiple instances of the application running making clobbering your preferences and caches really easy. I don't suggest using this.