Convert Internal to External HDD and Boot via USB?

This Christmas, I am fixing up an old Dell Optiplex GX60 donated to me by a friend and giving it to my cousins who desperately need a new computer. (I say "new" because I found out that they were running Windows 95 on their machine which is, understandably, dying). I have scavenged an old Apple eMac I purchased at a surplus sale and have acquired the internal HDD, in this case a Seagate 40GB model. Since they are only needing a computer for school work, this is not a major problem. There is, however, one problem; the computer did not come with drive mounting rails that I could use to install this HDD.

Is it possible for me to take this internal drive, purchase an external enclosure and have this HDD boot through the Dell's BIOS via USB? My thanks to anyone who can help answer these quandaries!