As others have stated get a external 1 or 1.5TB hard drive, the one I mentioned in my previous post would require the purchase of a enclosure. I prefer to get External HD's this way becasue I can then purchase the enclosure of my choice. I would get one that had USB as well as Firewire (you are protecting your investment this way). It is very simple to install a HD into a enclosure so do not be concerned.
Now as to the data, there are different things to consider one is a clone of your existing HD, easy to do more on that later. Or a backup of existing data, this could be simple if you have your data organized in a simple manner. By this I mean if you have everything in your "My Documents" folder you could simply drag that folder over to the new drive and it will copy it. IF you have not organized the data you create in such a fashion I would suggest you do so. I created a folder called 'Data" which has sub-folders by application such as "Picture", "Word", "Excel", you get the idea.
Now back to cloning if you by a drive from Seagate it comes with very good cloning software made for them by Acronis. This software is FREE with the drive (and only works if one of the 2 drives is from Seagate or Maxtor) and is very easy to use. You would start the application and follow the prompts for cloning a drive or you could even use the install new drive feature which the 1st time would do the same thing.
Now there was talk of SSD (Solid State Drives), in your case I would stay away from these for a number of reasons. First the older Dell you are using probably could not support one. Second, they are very expensive of a cost per GB basis., while this is decreasing it is still much higher than a spinning HD. Third, I am still concerned about the life expectancy of the consumer version of these drives especially when used as the only drive in a system. A large number of the less expensive drives to not have Micro--code that supports wear leveling, this may lead to Cell burnout and premature failure of the drive. But from a performance perspective the are excellent.