Adding a 1.0TB SATA drive as a secondary drive to my IDE primary drive: Doable?

I have a Dimension 4600.  It's about 7 years old.  It has a WD800BB hard drive (80GB) as the primary.  I want to add secondary hard drive so that my programs are on the primary and all the docs, etc., are on the secondary.  (80GB is plenty for my programs!)

My system uses XP Pro (Patch 3+).  It's all up-to-date.

Dell sold me a Western Digital 1TB SATA 3Gb/s Hard Drive 64MB IntelliPower 3.5IN Caviar Green for $64.99 (as yet, unopened!) and Dell assured me (repeatedly) that it is 100% fully compatible with my system.  I've begun to have my doubts.

I've spoken with various local techies and they paint a nebulous picture.  Various sources have told me:  (1) that I might need to upgrade my BIOS to use a SATA hard drive with my PATA, i.e., IDE, hard drive; (2) that upgrading the BIOS is iffy and could smoke my motherboard, necessitating my buying a new motherboard; (3) that I will need a conversion cable to connect the SATA to the IDE (PATA) hard drive; and (4) that I'll need a controller card for the SATA drive because it's unlikely that my system will recognize anything as large as 1.0TB.

Any insightful, savvy, experienced folks want to weigh in on this heart-wrenching problem?  If so, bear in mind that I've got a chance to buy two new WD 2500 (250GB) hard drives for $90 (2 x $45).  While it's less than half the space I'd get with the SATA setup, the issues are miniscule---or so it seems.

Yes, I'm a babe in the woods, a lamb who fears he's being led to the slaughter.  Help!  Thanks.

Need more details?  Let me know and I'll provide all that I can.  Best wishes.