Thanks to all for trying to help. I already knew about how to clone my drive (I have an EZ Gig III CD and enclosure kit that works wonderfully.) I did some more research and found out that my computer required an EIDE drive.
Fortunately, I was able to format the SATA drive I'd purchased and Best Buy allowed the exchange with no hassle. Purchased a EIDE/PATA drive, cloned my old drive onto the new using EZ Gig, and swapped them. It booted right up with the full operating system and all my programs intact!
I now have a 250GB drive installed and am very happy... though a little disappointed that the associate at Best Buy didn't know what he was talking about when he sold me the wrong stuff! (I had to exchange the original enclosure I'd bought, too, because it was the wrong type.)
Thanks anyway. Hope this posting helps someone else figure it out.