OK, I'm a lurker, but I know exactly what you need to do. But your emergency warrants my first post. Here it is, step-by-step:
1. Breathe!
2. Unplug the old, accidentally erased hard drive immediately (ETA make sure you do it properly and eject it and then unplug it- don't just yank the cord out in a panic-LOL).
3. Plug in and format the new drive.
4. Download the demo or purchase
Data Rescue II by Prosoft Engineering (You can download the demo free, and then use
these instructions to purchase if it shows you the files it can recover and it's suitable to you.)
5. Plug in the old drive that you accidentally screwed up and connect it to the computer
6. Do a Scan (have a glass of wine and relax while it does its job) on the old drive you accidentally erased
7. Recover files using the newly formatted drive (the one you originally intended) as the working volume. (Depending on how much it finds, and how it finds it you may have to do some cleaning up after it recovers the files.)
If you need any hand-holding PM me and we can chat online or on the phone.
Running the program may take awhile. The longest step will be recovering the files. Data Rescue has saved my @$$ quite a few times. It has saved me thousands in data recovery fees when drives have gone wonky (before I was backup anal).
Oh, and if you did a low-level format you're probably screwed, but if you just did a quick one, you're probably good. This is worth a try. It does have a last-ditch deleted files Scan in the program, but the other scans might recover it better if you can try them first.