Some Dells will not notice a HD until it has been partitioned and formated. I worked on my brother-in-laws Dell not too long ago and he had the same issue. What you have to do is just hook it up. Put in your Windows XP CD and boot to it, and it should recognize it there. If it does, just use WinXP to partition and format the HD but 'back-out' after that.
Also, sometimes depending on the mfg. of the HD. You can get tools from the site to partition and format the Drive. Maxtor has something like Maxtools or Max-something that helps. Also, Seagate has tools as well. Assuming you can get to a desktop wioth it hooked up.
Also, do you know if the drive supports 'hot swap'? A term I may have taken out of context, but it is when the HD can be connected on a running PC. Some SATA HDs allow this but I do not know about ATA.