If your data is imporant.... if you have programs such as income tax, photographcs, accounting, spreadsheet, word processing documents... it is wise to save them to a safe location before you start changing things a lot...
Not necessarily. The XPS R400 was a Windows 2000 machine. It was a rugged computer built for constant use in an office or business setting. But it was last built in 2000, so is 9 years old, and came with Windows 2000 instead of WXP. I don't remember the memory maximum for sure, but as I recall it could hold only 384 MB or PC133 RAM in three 128MB modules...
I have no idea what the hard drive would be... but you could save files to it while you work on the Dimension 2400.
Actually, the Dell Dimension 2400 is a reliable hard working computer that will accept 2 GB or memory... It is remarkably fast for a basic home computer... It is limited in power supply output. It is limited in video graphics. But it is a darned good computer if not pushed beyond its limits.