As someone else very validly said, the key is to go slow. So if you've still got that heavy-chest panic feeling go and take a walk to think it through.
Follow these steps before going down the expensive data recovery route. I've recovered literally thousands of files from my own and friends drives without using professionals. The only time I needed to send a drive off was when it physically broke - they had to rebuild it using an identical model = ££££.
Now I backup using Backblaze as-well as an external Time Machine drive which, 99% of the time, isn't connected to my Mac. Haven't lost a file in years.
1. Make a byte-accurate image of the drive using dd
1.1 If you use a data recovery program and it goes wrong you'll lose your work. Therefore, use another Mac and connect the Mac with the work on to it. You'll need to use FireWire target disk mode, if the machine has it (otherwise you're going to need to get a disk enclosure. These cost next to nothing on eBay):
http://www.macosxhints.com/article.php?story=20020401233536141
1.2 Then use the dd command to make an image of the drive:
http://www.macosxhints.com/article.php?story=20050302225659382
1.3 Then
lock the image (Click on the file > Get Info > Tick the 'Locked' checkbox.
1.4 Disconnect the Mac with your work on and put it somewhere safe. Not on the edge of your desk because you're in a hurry.
2. Work on that image, not the drive with your work on
2.1 The first thing to do is to see if the image mounts by double-clicking on it. If it does, simply drag your files/work across and don't make the same mistake again.
2.2 If not, make a copy of the image. Unlock this new copy, and leave the original intact.
2.3 Install one of the recommended file recovery programs on your temporary Mac,
not on the image with your work on - you risk overwriting something important.
2.4 Then run the file recovery program(s) on the new, unlocked, image. You'll likely recover all your work but often I notice that certain metadata, such as date modified, isn't pulled across. I'd like to think due to the very small amount of data you overwrote you'll be ok.
2.5 If those don't work (which would surprise me very much), then it's time for data recovery.
Good luck. Any more questions don't hesitate to ask. And don't worry about all the pointless abuse you've received here; despite you already admitting your fault some people will always feel the need to go high and mighty, explaining how they'd
obviously never do something like that. Everyone make mistakes.
AppleMatt