If you have a laptop you may want to keep it. In the event that the lappy hard drive starts acting funny and doesn't boot, you can use the 2.5" that you just got to hook to your desktop, run scandisk, reformat, etc. That's what I did with mine and it saved me a couple hundred dollars of repair/replacement bill.
Daisy chained drives will definitely not fit in a 3.5" enclosure. Maybe 2 HDD could fit in one enclosure designed for a CD-ROM, which is what I have. Simply get an IDE cable with two drive hookups, plug it into the enclosure, and plug two drives into the cable. Master/Slave settings still need to be set (if CD-ROM and HDD at the same time, HDD needs to be master), and there might be a few performance anomalies like if you have a DVD burner and HDD at the same time, the burning might not go so well if you're transferring to the HDD hardcore.