Hi,
I just finished upgrading to Windows 7 64-bit from Vista 32-bit (which came with my machine) on my Inspiron 1420, by using the custom installation from a Windows 7 Home Premium Upgrade disc. I was kind of intimidated about necessity of 64-bit driver since there is no 64-bit support option on Dell's website. So having gone through it successfully now, I thought I would share how I did it for those who may be thinking about the upgrade.
Chipset, Audio and 3945abg Wireless:
Although Dell does not list support for 64-bit drivers for the Inspiron 1420 on its driver homepage, there are three drivers on that page that do support both Windows Vista 32-bit and 64-bit. These drivers are for the Intel Chipset, Sigmatel Audio and the Intel 3945abg wireless card. Note that the Inspiron 1420 booted fine for the first time after the Win7 OS was installed, and the wireless card and audio were functioning with the Win7 native drivers. So the Dell drivers are not absolutely necessary. However, I decided to install the Dell chipset and wireless drivers from the 1420 driver page in case they would be more efficient.
Touchpad and Quickset:
Now, Dell's 1420 driver page does not have 64-bit drivers that may be used for the touchpad, quickset functions (external audio buttons) or video card. After booting Win7, I found that my touchpad lacked the scroll features and only the mute button for quickset worked. However, no quickset graphic popped up when the quickset buttons were used either. Fortunately, Dell provides touchpad and quickset drivers for Windows 7 64-bit for the Inspiron 1545. I downloaded and installed these drivers with success. As far as I can tell, the scroll features and external quickset buttons function fully with the 1545 drivers.
Video Card:
Finally, the video card. I have the NVIDIA 8400M GS. In the past, I do not believe that the nvidia website offered support for this card. In any case, there is a Windows 7 64-bit driver on their website that supports this card and worked for me. Note that the nvidia website recommends updating the bios of the computer to version, A09 (available on Dell's driver page). I had updated to the newest bios, version A10, and the driver seems to be compatible with this version.
For what it's worth, I also installed 2x2GB (for 4GB total) of DDR2 RAM prior to the install, and the OS recognizes all 4GB.
I hope this helps some other Inspiron users who may be interested in upgrading their OS. Although I have not used the OS a whole lot yet, the machine seems completely stable and functions well. Good luck!