I upgraded my 420  to win 7 before A07 came out, so you can update it after the new OS is installed.  These instructions might help you with a clean install on the new drive. Win 7 will let you format the new drive when you boot to the dvd and select Custom Install.
  Why is the A 07 bios necessary before upgrading the 420 to Windows 7 64 bit??? My hard drive recently died (Vista Ultimate) and have a new hard drive coming in a day or so along with am upgrade version of Win 7 Pro 64 bit.  Since I'll be installing a new WD 1 TB SATA II drive before clean installing Win 7 Pro, how do I upgrade the bios on a non functioning, non bootable computer prior to adding the Single new hard drive and then doing the clean install upgrade from Vista? Help please!! I want to do this upgrade Wednesday night..... Thanks 
XPS 420 Do I need to go to bios A07 before clean install of Win 7 Pro 64 bit? 
How to do if system in nonbootable now due to hard drive failure?