Actually my Inspiron 6400 which is the same model had Vista Home Basic on it, most models were customisable to Ultimate and Home Premium.
You can verify by checking the COA at the base of the laptop if it says Windows Vista Ultimate of another version of Vista or XP.
You can also use the Digital River setup files and Activation Backup Recovery or inputting the key on your COA and phone activation for an OEM licence (preinstalled Vista Ultimate). For an Upgrade Product Key you can obtain it from Belarc Advisor and use the Upgrade Product Key with Internet Activation.
As you are in the states you are best to fill out this form and get the media direct from Dell. The only reason I am recommending this over the method mentioned above is because this system is a Media Direct system and works best if the Media Direct Partitions are setup, Windows installed, Drivers installed and then the Media Direct Application installed otherwise boot issues can occur if the Media Direct Button is pressed instead of the power button. To do this you neeed a Media Direct DVD and a Windows DVD.
Follow the Steps in my wiki Windows Reinstallation Guide/A Clean Install of Windows Vista:
http://philipyip.wordpress.com/dell-community-forums/