I have a Dell Precision 390 and Windows 7 Ultimate 64-bit loaded just fine and it's up and running. It's been strongly suggested though, that you do a clean install. So I wiped my HD (after backing up personal files). Don't just format your drive, I mean wipe it totally. Automatic updates will install the drivers you need for any attached device or go visit the manufacturers for your drivers. So in short, "Don't sweat it". Go ahead and get that puppy going for a few more years.
We have a Dell Precision 390 and want to upgrade this to a Windows 7 64-bit operating system.  Several months ago I noticed driver support for Windows 7 64-bit for the Dell Precision 390 model but currently the driver and downloads site is not supporting any such drivers for Windows 7.  It seems the support has been withdrawn.  At best,  it supports only  Windows Vista 64-bit.  Can anyone elaborate on why there is no driver support for the Dell Precision 390 for the Windows 7 64-bit platform? Tony  
Dell Precision 390 and Windows 7 64-bit Driver Support