Worth trying/purchasing data recovery software?

Hi, I am thinking of purchasing one of these products but have a few questions.

My situation is that I had some photos from my digital camera which I put on my mac - into iPhoto. I deleted the full camera card and have since used half the card or more for new photos. I then accidentally deleted the files from my computer. I still have the thumbnails shown in iPhoto but not the whole files (I think I deleted all the originals and data files). The hard drive is almost full (and probably was when the photos were put on the computer). Im worried that on both the camera memory stick and the hard drive of my computer, that the data has been overwritten.

I also have some burned DVDs and picture discs that seem to have become corrupted and Id like to salvage the files.

Would you say that there is still a decent chance that the data can be recovered? Once data is overwritten is it impossible to get the files back? If not, how many times does it need to be overwritten to lose the files?

Sorry if the questions are dumb - i really know nothing about these issues...

Thanks for any help...

Once the files have been overwritten there is a slim chance of still recovering the data.

However you need to do this before you put anymore onto the disc or memory card. Then run the software from an external drive and save any files to the external drive as well.

Good luck.

Ditto the above posters, when you delete a file the data is still there but you need a recovery program like Data Rescue:
http://www.prosofteng.com/products/data_rescue.php
Do not install it on the hard drive! Once data has been overwritten it gets more difficult to recover and even if the file can be found, it might be damaged.