New HDD ??

The 6400 will use any standard 2.5" 9.5mm SATA drive as ejn states.  Any adapter can be removed from the old hard drive as I've been doing with my old Inspiron 5160 for six years now.  You can purchase the drive from Dell if you wish, but you will generally pay more for it.  Dell buys them just as you can, only they usually buy in large quantities direct from the manufacturer.  They don't generally pass any volume discount they may get to the purchaser, however.

Seagate, Western Digital, Hitachi, Samsung, Toshiba, and I've probably left a couple out, all sell hard drives for laptops.  Check out this page:  http://www.newegg.com/Store/SubCategory.aspx?SubCategory=380&name=Laptop-Hard-Drives

Saving your data and reinstalling from scratch is not a bad way to go.  I've done it many times over the last twenty years on various computers.  A clean installation gives you a fresh configuration that isn't littered with the remains of many programs that have been installed and removed over the years, and a clean installation generally runs faster than one that has been around longer than a year or so.  This is usually a straightforward process, but it does require considerable time since you must install the os, drivers, and any applications you wish to continue to use.

You don't absolutely have to do it that way, but to transfer your existing system to a new hard drive requires a program to transfer the image (Acronis True Image and Symantec Ghost are examples) plus an external drive shell to mount the new drive.  If you purchase the drive from a store such as Staples, Office Depot, or Best Buy you might even find that the drive includes a CD having a transfer program on it.  Seagate and Western Digital will, I believe, allow you to download from their web pages a trimmed up version of Acronis to use if you have purchased one of their drives.  If you purchase the drive from an on-line store such as Newegg, however, you will usually receive a "bare drive", meaning that you get the drive with nothing else in the package.  It's cheaper that way, though.  The main expense other than the new hard drive will then be just the external drive shell that you will attach to the laptop via the USB ports.  The external drive shell needs to support 2.5" SATA laptop drives.  You can find a sampling of these on Newegg as well.  Just be sure to select one that indicates it supports a 2.5" drive as there are also shells for 3.5" desktop drives:    http://www.newegg.com/Store/SubCategory.aspx?SubCategory=92&name=External-Enclosures

Please don't think that you must purchase from Newegg; I refer to their pages simply because I purchase from them when it's convenient and I remember their web page address.  There are other on-line stores that I'm sure will treat you just as well.

Thanks for clearing that up. I, for one, thought that it applied to both type of drives.